First mistake was letting them touch your car. I am shocked that you would do such a thing. I know "they are supposed to know", but more electrical issues stem from crappy stereo installs, than any other related problems.
I would disconnect both + and - on the battery for 30 minutes. Then, reconnect it and make sure the terminals are corrosion-free, and tight.
Then, I would make my way down there, and ask the store manager what he intends to do to fix your car now that it is broken.
having an electrical problem is no joke. If it truely was "fine" before they touched it, then why would you assume it was just a coincidence.
I walk past a "worst buy" every day to get coffee. I see BRAND NEW cars in there getting install's done...and you can see the 18 yr old kids/installers, joking around, and having "whip" fights with strands of wire...not the kind of place I'd take my $50k brand new car(or my "used to be $40k Lincoln")
Quit throwing "guesses" at your car, and either take it to get it scanned, or let someone who knows the EEC-IV/OBD-I system.
I know of 4 vehicles that have had to be completely re-wired, and new ECU's installed from shotty car-audio installs. My GF also works at a dealership, and they see brand new cars come in to be repaired. One such car had its warranty voided, and the owner had to spend $10k to have every wire on the car replaced because some moron installed the stereo, and cut/spliced into the wrong wires, frying the entire ECU/electrical system.
not to scare you, but...this is serious business. If you dont know what's wrong, quit throwing parts at it.
