Keyless entry late W126 - illustrated
I have illustrated the installation process for the Crime Guard REC-11 (available from Crutchfield.com, together with lousy technical support) on my web site. Go to Cars - Technical - Chassis.
This only applies to later W126 cars where central locking can be actuated from the trunk with the factory mechanical key. Has worked satisfactorily on my 1991 560SEL for 2 years now.
Do this on an earlier car - like my 300SD (1983) and you fry the vacuum pump, door lock actuator, or both. Don't ask. The factory chassis manual incorrectly illustrates the driver's door lock as using vacuum. It does not. It is merely an electrical switch, so vacuum supply pump activation cannot lock the driver's door, the circuit (which looks for no air passage to switch off the pump) continues to cycle, and things fry. The Electrical Troubleshooting Manual for my 1983 illustrates this correctly (i.e. no vacuum to the driver's door) but by the time I found that out it was too late.
Anyway, one pump ($85 from the wrecker) and one actuator ($25) later, I have yet to install remote locking on the 300SD, which will require a separate actuator installation in the driver's door. Stand by....
While I did not illustrate the installation of the extra horn, on the 560SEL I affixed it to the underside of the storage cubby on the right side of the trunk, where it is exposed to the elements. A neater alternative is to use the car's horn, but I did not want to bother with running another wire. You also need a switched power source for programming - as I recall I found one going to the pump in the 560SEL. For the 12v 'on at all times' source I tapped into the feed to the trunk light before the switch.
OK, so it's a work in progress, but I thought the wiring diagrams might help someone.