It definitely can be done with two simple relays. I did it on my 1988 300e.
The lock wire is in the driver's or passenger's side kick panel. It is either blue or green depending what side. At least this was the case with my 1988 300e and on my old 1985 190E. Hope this helps you or whoever does your alarm. It's too bad that most stereo shops don't know what they are doing, so the customer has to babysit them.