My wife had expressed a desire to have keyless entry on her '86 560 SL, similar to that on the SDL, so as a surprise gift I installed one.
This is fairly easy, as it turns out. However the only feature I enabled was keyless entry. Nothing else was hooked up. No alarm, no interior lights, no starter kill or trunk pop. Just keyless entry/exit, with indicator lamps. Good enough, if you ask me. Unlike the earlier Audiovox unit installed on the SDL, this later unit requires the (included) siren to be installed. However, it can be programmed to not chirp for lock/unlock, and as the alarm features aren't connected the only other time it makes noise is during programming. I hate noise.
This installation can be done entirely within the trunk. First, procure a suitable keyless entry system. For about $48 (each) I purchased a basic 3-channel 2-button Audiovox APS-25CH system. This unit was chosen because it is capable of locking/unlocking from a single button. This, in conjunction with its basic learning ability, multiple channels, and the two buttons on the fob, allows for operating up to three cars from the same fob, a very desirable feature for us. Audiovox's web site is very confusing, and it is unclear whether or not very many of the multitude of keyless units they make is capable of this. (The older APS-45 [SDL] was explicitly advertised as able to do this, as is the APS-25CH and maybe one or two others.)
The APS-25CH, like others in their line, has an H-bridge type of drive for the locking system, it's intended to drive a motor one way or the other for the typical electric lock system. (By H-bridge, I mean that it has two terminals coming out of it, and each terminal is high-impedance normally. For locking one terminal goes to +12V for 0.85 seconds, and the other goes to ground. Vice-versa for unlocking. Current output is 300mA or so. Perfect. These are ideal for driving an electric motor, which is the common type. It also has a bazillion other wires, most of which are irrelevant for this application.) This unit also has a setting for MB/Volvo locking systems, whereby the time it drives the motor terminals is extended from 0.85 second to 3.5 seconds. This setting is crucial for making it work, btw. You'll need to make sure your unit has such a setting.
In fact I bought two such units, with the intent of eventually replacing the unit in the SDL with a new one. This will allow both cars to be operated from the same fobs. (It is unfortunate that the APS-45 and APS-25CH can't learn each others fobs, but I suppose I can find an eventual use for the old APS-45.)
Anyway, behind the battery in the trunk of late-model 107 cars lives the vacuum/pressure pump that operates the locks. There is both a 2-wire and a 3-wire connector on the pump itself.
The 2-wire rectangular connector is power/ground, into which I tapped for the remote's red and black power wires. (The brown wire is the MB's ground.) The round 3-wire connector is the switches from the three keylocks. In the harness next to the pump is another round 3-wire connector pair. One side has three individual wires (blue, green, and yellow), the other has a single cable. Cutting the individual yellow wire isolates the trunk key switch from the pump. Connect a 100 Ohm 2W resistor between these two now-loose wire ends. Connect the appropriate locking wire from the remote unit (red, in the case of this Audiovox) to the side closest to the connector. That is, not on the side that the trunk switch is. A voltmeter can verify you have the correct side. (The trunk switch lead alternates between +12V and ground as you operate it.) I found that once you disrupt the yellow wire, the pump will not operate correctly until it is unplugged and plugged back in. Make sure you do this. Tuck the keyless unit and its siren into the unoccupied corner behind the battery box and voila, you are done!
On a '88 560SL, a friend found that the second connector in the harness that is tapped for the resistor is no longer there, so he had to go back to the trunk actuator itself to make the tap. Just a bit of extra work opening up the back wall of the trunk, nothing heinous.
Well, not quite yet. Experience with the SDL showed that the lack of indicator lights was a real annoyance, and this time I determined to do better. The white wire from the keyless unit is for flashing the lights. On most MB's, the parking light circuits for left and right are separate. Originally this separation was for the night parking light settings of the light switch, although even without this feature the later models' lamp-out circuitry would have necessitated this. Regardless, the single wire from the keyless unit is unable to flash the lights on both sides without a little help. Two diodes from the junk box were enlisted to keep the circuits separate. The cathodes (non-bar) were connected to the white wire, and an anode (bar) from each diode was run to the rear parking lights.
Connection to the parking lights was accomplished by opening the connector shells of the plugs to the rear lamp assemblies. (Care should be taken to not mix up the pins while the connector shell is open. For safety, I removed the appropriate pin from the connector shell and then put the shell back together temporarily to keep things in order during soldering.) Pin 6 (the third pin down on the right-hand side) is the parking lamp. I unsoldered the pin from the wire, blew out the solder, and then soldered in an extra wire to the pin. This wire was run to the appropriate diode. The wiring is tucked behind the carpeting, and is invisible.
Before putting the right-hand lamp assembly back, I tucked the antenna wire into the space behind the reflector (non-lamped) slot in the lamp assembly. The SDL's antenna was too 'buried', and you have always had to be very close to the rear of the car to operate it. With this greater exposure to RF (the lens assembly is plastic, and RF-'open' to the rear of the car), I anticipate the operating range of this unit to be greater. (As it has proven to be.)
Now to program the system. The default 0.85 seconds lock time is not enough time to cycle the system. The unit can be reprogrammed for a 3.5 second output, which is sufficient. But to do this you need to conduct an elaborate dance with the system's valet switch and the `ignition' switch, which I have not connected and have no intention of so doing. Instead, I connected the yellow `ignition' wire to an extra switch from the junk box, with the other end connected to the red power wire. The valet switch and LED (included) are tied in the bundle next to the unit. Reprogramming is then straightforward, though irritating, according to the instructions in the manual.
I also programmed off the lock/unlock chirp, and taught the unit to lock/unlock with the second (small) button from the two fobs for the SDL's new keyless system, ready for when that is installed. (This makes four fobs that will operate this unit, which fills its internal tables.)
The basic theory here is that the remote unit `overrides' the input from the trunk key switch. The 120mA current (limited by the 100 Ohm resistor) through the switch should not be enough to damage it. When idle, the remote unit is high-impedance and is effectively not there.
By using the trunk (yellow) wire, the remote also arms and disarms the car's factory alarm system. (The doors have separate alarm switches in the keylocks, so that locking/unlocking from inside the car doesn't activate the alarm. But the trunk doesn't need this, so the single connection activates both functions.)
This installation is about as seamless as it can get. You can lock/unlock, arm/disarm the car either with the key, or the fob. Your choice. In the worst case you have to hit the button twice, if you've used the key to leave the system in the opposite state from the last time you used the fob. There's no issue of having the car alarm go off if you unlock it with the key, as can often happen with an add-on alarm system.