And call a good lawyer who deals with consumer issues. Also, if the shop is a Garmin dealer, detailed complaint to Garmin.Very sorry about the ordeal, I’d look for an exit of some sort. I imagine it’s not easily flyable now.
My panel won’t win any awards, that’s ok with me. Back in the day I flew to Alaska with a boat Loran(where it worked), a weak VOR, and looking out the window. Now I get by with a newer KX-155, GPS-175, 2 G-5’s, and my IPad Mini, tied to the GDL-50.
I’d tell them you want the aircraft flyable, to leave, then bring it somewhere with better reviews.