Mercedes uses unusual center door lock, there is only one wire in each front door and trunk which controls the center door lock, when this wire has 0 voltage (Grounded), it unlocks all doors, when it has 12V voltage, it locks all doors. when it doesn't operate, it stays with high impedance. Please note, it doesn't require a constant voltage, all it needs is a longer than 2 second pulse to allow center air pump under the rear seat to finish the operation.
So the object of the add on remote control, is to ground that wire for about 2 seconds when "unlock" key is pressed, and to connect the wire to 12V voltage for 2 seconds when "lock" key is pressed. After that it should maintain the original connection, one may still be able to use key to open all door.
If you ordered the 3 button Valiant remote control, it has 3 relays, one closes for about 2 seconds when "unlock" button is pressed, and one closes for about 2 seonds when "lock" button is pressed, the 3rd one is for remote trunk release which you don't need. You can not just simply use one relay to ground it, and use the other one to connect it to +12V, because if you accidentally press one button right after you press the other one, you will have a short circuit.
Because Mercedes is unique, you need to modify the PCB board for the target above. I have attached the schematic from mbcoupe.com, the author was very smart to put two relays in serial, it doesn't interfere the existing mechanism at all, and there is no risk of short circuit.
It'll also be good if you install it near the air pump, only the control range may be less due the shielding caused by metal body and tint glass.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.