I have a tube I'd like to stick under the seat of my '01 S-crew. I have speaker level inputs for the amp. Whre is the best place to grab it from? I'm thinking the wireing going to the rear door speakers but could not figure out what was + or -. I have grey/blue stripe and tan/yellow stripe at the speaker on the drivers side; back/white stripe and orange/red stripe on the passanger side. How can I tell what is what?