It is really back there.
Those two wires that plug into the back of your instrument cluster connect to the combination meter. A LOT OF STUFF is contained inside of the combination meter such as controls for your AC, ABS, Headlights, Airbags, and pretty much every thing electrical. You could have damaged the circuit while you had the cluster out.
Just recently I installed an alternator pulley, and somehow, my AC amplifier blew out. I have no clue why, but it did. It most likely wasn't caused by you tampering with the instrument cluster, but it could have been. I'd take your gauges out, and just leave your factory ones in and take it to another dealership. That should work. Maybe.
EDIT: Either way, let us know how it works out, and PM me if you have any more questions. I am in the process of trying to fix my AC amplifier right now, but I have no idea how to do so, so please, if you find anything out, please let me know.
Ask him how he fixed it if you can.