2 Head units down the drain

RonRizz

Deliriousclam hit the nail on the head. They are trying to put a high level signal into an amp that is probably less powerful than the head unit itself. This is why you have so much system noise (hiss) and the audio is blaring at low volume settings..... Any shop that cannot perform this task would not be anywhere near my audio system. I may drive by and let them look at it, but they wouldn't be touching it.
The shop should also refund your $$ for the install. How could they turn the car over to you like that in the first place? I'd be HOT.
by the way, I hope the $1000 you quoted was an exaggeration......unless it included the head unit..

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