I made two adapters so I can use an aftermarket amp with the factory harnesses. The adapters basically plug in where the factory Haes amp would plug in.
Amplifier harness adapters
I used a donor Haes amp to make my adapters. I think I paid less than $20 for the donor Haes amp on ebay. Either making an adapter, or just running new wires, is so much better than cutting the factory harness, IMHO.