Sounds like a crappy Tech to me. Most dealers will try to void warrenties at the drop of the hat (cause they don't make money from related work). As long as your install has no relation (and it is the burden of the shop to prove otherwise) to the issue at hand, there is no issue.
I've been through this many times on my old Jag (wheels and brakes vs electrical issue) and even recently on my Escape (HU and Amps vs manual temp adjustment issue). The first thing a bad dealer wants to do is blame the changes, but if you argue with them enough they change their tune. It's frustrating though.
There are two things I would suggest if you are worried about this. First is to have a pro wire up the power for the 1900 (had a terminal block added with my last Amp). Second make sure that it is done in such a manner that should you have weird electrical issues you can disconnect the Mac and related items and reinstall the original hardware (not as a cover up, but so you can see if it is still happening so you can tell the dealer where to stuff it when they try to blame your install).