Is there a wiring harness available to add a powered subwoofer to a stock 2007 F-150 head unit?

 

A friend of mine just did this.He went to the local audio shop and asked for a hi-low adapter.It lets you "tap" into the speaker wires behind the radio and converts it to RCA jacks.He then ran RCA cables to his amp.He paid under $30 for the adapter.He did have to adjust the adapters gain screw to get the sound level the way he wanted it.