I installed JCwhitney Valiant remote controller on my W124( 92 400E) 2 months ago, it's running perfectly, I got the idea from mbcoupe.com:
But I got the cheaper version of Valiant, it has only 2 buttons(lock and unlock), the original valiant system described in mbcoupe.com had 3 buttons, the 3rd button was wasted(for trunk release).
Because my 2 button Valiant has different PCB layout, I did figure out the proper connection by a multimeter and cut some tracks on the PCB, also soldered some wires on the PCB, then I also mounted the receiver on the back of tail light.
By using exact the same remote controller as described in mbcoupe.com, you just need to completely follow the instruction. For W126 and W124, the color code for the wires may be different, the control line is yellow, the +12V may be Red or Red/White, the Ground may be Black or Brown. It's good to verify it with a multimeter first.