Check my website
Wiring, pictures, you name it, everything you need is on my web site - you should follow the late W126 version. I disabled my alarm in the 560SEL (two plugs in the passenger footwell) as I like the 'lock on start' feature but that sets the factory alarm off.
The early W126 has a different circuit, so make sure you look at the right pictorial. Your installation will be in the trunk where the vacuum/pump for locking/unlocking resides. It's in the right rear corner of the wheel well, where the spare is.
I go the whole hog here with horn honks and flashing lights - you don't have to do those, but they are nice to have.
Go to Cars - Technical - Chassis.