You'll need to use the preamp that's on the back of the headunit now, or use something with a high level output as the factory amps expect. It's somewhat above line level.

Then again, the factory amps are kind of junky... I'd just replace everything.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.