Here's the gist of my email, so others might share:
I've had many, many emails on this subject. Now I know why the guys at Mercedesshop quit selling this! MOST people have had no problems installing theirs. I've had a few with older W124s tell me that the wires were different, the alarm didn't work right, etc. For those, I'd suggest that MAKE SURE you follow the directions regarding testing the wire with a voltmeter. Some have reported more than 1 blue wire. Also look at the instructions from Mercedesshop:
This shows how to work with the convenience control module, which is where the alarm is. Mine and many others of similar vintage (92-94) has no alarm problem and I didn't wire into this module. Alarm works fine with or without the key or keyless entry.
The last suggestion is to keep after Crutchfields. They've sold a ton of these things to Mercedesshop members and should have notes on the differences. Sometimes they have to do some digging...