what kind of alarm are you buying?
I know when I bought my DEI keyless ($60) from circuit city it had onboard relays for locking/unlocking the doors I had to purchase additional relays for the horn / lights...
This is the wiring for my keyless "Valet Model 712T"
Due to the fact that my 300SD is a 1st gen w126 and only had 1 key hole that would unlock the entire car I had to add a 12v actuator. If I didnt I would only be able to remotely unlock the 3 doors/trunk/gas tank.
In a 2nd gen w126 this shouldnt be needed. One could just wire it up to the vacuum pump. danwatt over at BenzWorld.org did this, its what inspired me to do mine but I discovered that I needed the actuator.
I also plan on installing some kind of alarm/keyless in my 93' 190E...I also plan on installing a independant killswitch. I think that is the best deterrent of theft. My friend wants me to put in a killswitch system (consisting of 2 switches that are hidden and one push button that must be held in order to start the car).
oh, ha i seem to have BS'd a bit, here is that pic:
(its rather huge so I've just linked to it instead)
http://i98.photobucket.com/albums/l250/MafiaNicky/64cf673d.jpg
lemme see if I can remember what I did:
i think it was something like this diagram..i belive the colors matched...
you might have to do somthing along these lines:
i'd say just experiment outside the car and get it working before you go attacking the german wires! lol
Kris