Audio and video installation service near Glen Mills, PA

Took my wife's car in to have a head unit installed. Simple install. Nothing complicated. When I picked up the car, the AIRBAG Warning light was on. I called and the guy told me it just needed to "cycle itself" because he had to unplug the warning light. Got home and looked at the invoice and noticed that he charged me $150.00 for a steering wheel control that Walmart and Amazon both sell for $28.00. I trusted that he wouldn't gouge me so I didn't ask the price. 4 weeks and 1500 miles later and the AIRBAG light was still on. I was about to trade in the car so I called him and let him know. His only response was that it should've recycled and to take it to the dealer. No mention of him paying to fix what he caused, etc. The dealer quoted me $150 just to look at it. Everything I read online said it was a seat sensor that can be blown if wires behind the dash get shorted. When I got there to trade the vehicle my value was lowered by $500.00. This guy's error cost me $500.00 in trade value on my car. I'm filing in District Court today to at least get back the money I paid him to do the install. I'm out over $750.00 but all I want back is my $250.00 for the install. I hate being ripped off. People need to take accountability for their mistakes.