Originally Posted by scottmcphee

More on this method...

Here are the wires to go after in the car trunk near the latch (1987 300D)

Red/White or maybe just Red is HOT +12 all the time, from battery.
Brown/Blk or maybe just Brown is GROUND all the time.
Yellow is the input to the computer.
Supply a +12 pulse on yellow (taken from the red wire is fine) for UNLOCK all doors.
Supply a GROUND pulse on yellow (taken from Brown wire is fine) for LOCK all doors.


Try supplying the brief pulses on the Yellow wire MANUALLY by touching it for half second to the mentioned supply +12 or GROUND. When that's working, wire up your remote receiver relay to give these pulses when using the remote button.

When you release the button make sure the remote receiver is releasing the relay so the pulse comes off the Yellow wire so it is allowed to hang "free" again. Some remote receivers have a "latching" feature, don't pick that, use the "momemtary" feature... in other words the relay only holds for as long as you are holding the remote button.

The EMT diagram shows the Yellow input "resting" on +12 (Unlock). That's just a convention for diagramming, don't actually do that. Let Yellow hang free when neither remote button is being pressed, offering no signal to the computer. In order for you do this, you need a 2-channel remote, with 2 relays. If you try to use a 1-channel remote with 1 relay and just take off from its N.O. and N.C. contacts you won't be successful. (That was my first try).