AudioControl LC6i (Gray) 6-channel line output converter for adding outboard amps to your factory audio system at Crutchfield.com
Looks like it's cheaper now. That will allow you to keep your factory head unit and run 2 amps, one for your sub and the other for the rest of the speakers.
There are other alternatives but this was the direction I went. I didn't need to find a remote wire or anything, just a constant 12 volts and a ground. Tap into the rear factory speakers and you get 3 sets of RCA's to choose from. 1 dedicated sub amp and maybe a 4 channel amp.
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