My wife had expressed a desire to have keyless entry on our '86 300 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 or trunk pop, no indicator lamps. Just keyless entry/exit. Good enough, if you ask me.
This can be done entirely within the spare tire well in the trunk. First, procure a suitable keyless entry system. For $54 I purchased a basic 3-channel 2-button Audiovox APS-45 system. (Their install guy wished me luck. Ha!) This unit 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 +12 V for 0.85 seconds, and the other goes to ground. Vice-versa for unlocking. Current output is 300 mA 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.
Anyway, inside the spare tire well of 126 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.
Early 126 cars have a different system using only three wires total, but I think they can be made to work as well. There's no factory alarm to worry about. The three wires are power, ground, and the switch from the driver's keylock (only), which can be used in lieu of the yellow wire mentioned later.The 2-wire 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.) In the harness next to the pump is another 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 2 W 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 +12 V 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. Screw the keyless unit to one of the unoccupied corner walls of the tire well and voila, you are done!
Well, not quite yet. 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, as I'm not going to use the parking light outputs, horn outputs, etc., I don't need the system's second fused red power wire as such. So I connected the yellow `ignition' wire to the fused side of the extra power lead, and now I can just plug in the fuse to make `ignition' be on. Or off. The fuse I keep tucked into the wiring bundle for storage. 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.
The basic theory here is that the remote unit `overrides' the input from the trunk key switch. The 120 mA 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.
To add to the fun, I bought a Sesame Street "car keys" toy and gave it to her instead. "But honey, didn't you say you wanted one of these?" Let's go Elmo! The real fob was clipped to her keyring where she found it later. (She gave the Elmo thing to our young son.)
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.) I know this works, because on one trip my wife gave her keys to our son, and he was busily locking and unlocking the doors. What a hoot, he thought. Until I hit the brakes for a corner and the horn started beeping and the lights were flashing! He'd armed the factory car alarm. I made my wife take away his keys and hit the button, which shut off the fireworks.
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. The only nice addition would be to connect the parking light output, so that you could get visual confirmation of your request. (Waiting to watch the lock knobs move is slightly annoying.)
As one of Audiovox's APS units had already been installed, installation of the APS-25CH went much quicker. Removal of the existing APS-45 was merely a matter of removing the two mounting screws, and cutting the two red and single black wires that had been used for installation. The APS-25CH was mounted in its place, and the three wires were connected back up. The (mandatory) siren was tucked into another open corner of the spare tire well. (You can't successfully program the unit without it. I hate the things, but at least it can be programmed to normally make no noises.)
As mentioned in the installation for the other car, not having the parking lights flash had turned out to be more of an annoyance than had been expected. Also, the range of the fob was inadequate. This latter problem was addressed by poking the antenna wire partway through a hole in the tire well, so it dangled outside a ways. The lights were addressed exactly the same was as for the SL: two diodes (cathode [non-bar] side) were connected to the unit's white wire, and an anode from each diode was run to pin #6 of the taillight connectors to pick up the parking light circuits. (See the other car's installation for more details.) This wiring was concealed inside the trunk rear wall, and under carpeting. Some care was taken to avoid routing the wire where it could be pinched.
At this point, programming of the unit was necessary. It is mandatory to switch to the 3.5 second lock/unlock time. I also turned off the lock/unlock chirps. I taught the unit to respond to the small buttons of the other car's fobs, and vice-versa. My wife's keyring carries the primary fob for the SDL, so that the big button locks/unlocks the big car, and the small button locks/unlocks the small car. That seemed easiest to remember.
The still-functional APS-45 unit was put back into its box, for (perhaps) later installation into some other car.