In my years in the service dept. or parts dept., I can honestly say that this problem is kind've two fold. Many blame that the service dept. pushes the tech's to get the job done as fast as humanly possible while still charging full length of time (according to ALLDATA's labor rates) to increase profits, but you have to remember that that service writers are being pushed by customers wanting stuff done last week. And you have to admit alot of people wait to take their vehicles in till its on its last leg.
As for stereo/tint shops, most of these guys are rip of artist, and like someone else mentioned these guys hire young kids with no experiance. Car stereo's are easy nowadays with the plug-n-play harnesses; also Crutchfield has some great tech dept. that they offer with their purchases. As for complex wiring or specility wiring I'd suggest an auto electrician, you pay alittle more per hour but its worth it......