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First, make sure the line inputs are the same polarity. Next, if your amp has a switch for phase, try that; if not you can reverse the leads going to the sub. That's the best I can do unless you can post more info / pics.
There are several relatively cheap aftermarket double DIN head units for less than $200 on amazon or ebay.
Tying into the door speakers wouldn't be any different from the rear speakers.
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