I plan on wiring the F90BT myself and it seems pretty straight foward. However, my OEM HU has a fifth channel with an internal amp to power my 6x9 subwoofer. (6 disc headunit puts out 9w RMS on the four main channels and 18w RMS on the dedicated sub channel. )

 

Is there any way to wire this and keep this sub working with just the F90BT and a metra wire harness?

 

Thanks