I installed a Pioneer head unit with an HD adaptor and it uses a wiring harness that connects direct to the factory amp and runs it just fine. If yuo buy from Crutchfield, they provide the harness as part of the purchase and along with the very nice instructions, it makes it a truely Plug-n-Play install.

The instructions include a very clear note about swapping the 2 wires. (12VDC on all the time and 12VDC switched).

The upgraded headunit made a remarkable difference/improvement to the sound system!