I plan to install a 4 channel amp this weekend using the Preamp outputs on the stock Alpine head unit. However, I've never dealt with a car with a stock Amp and I'm not sure how to bypass it to power the bottom speakers from the new Amp.
Any help would be appreciated.

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only the bottom woofers in the door are amplified.. i recommend just running your own wires through the door boot... of if you know what your doing find the output side of the factory amplifier and locate each set of speaker leads (generally twisted) test them, the woofers are 4 ohm so if you have a meter you'll be able to find them easier

also, using the rear preouts on the back of the radio wont give you full range, its a subwoofer preout

 

I have the Alpine headunit with 3 sets of preamp outputs, not the Pioneer that only has sub outputs.
It seems weird that they'd put preamp outputs for a 4 channel amp on the head unit but make you bypass the factory amp to use them.

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