Perhaps if you attatch a microphone to your mufflers and then feed it back  into your amp?

I've never figured out the logic of sticking an amp and woofers into any car, let alone a pickup.....unless one's goal is to irritate the neighbors 

Lol, thats not my goal. I knew a kid who managed to get the frequency down so low, the neighbors couldn't hear the sound on some parts of some songs, would not register with human hearing. but they sure would feel it from inside their homes as he drove past. many would mistake it for a clap of thunder..  I used to love sitting in his car and just feel the bass. It was the weirdest sensation. He had a crossover and could eliminate anything higher than 15 hrtz. but at that setting most of the music never made it to the speakers, only the really low notes. Then with all that power driving it, when a note did finally come through, you knew it big time.

As far as mine goes, I made some progress in finding out some things. The Ranger was made for door speakers only, Someone who had the truck before me put a stereo from a windstar in it. Theres a place for another plug, but no harness in the dash to match it.

The experts say (men at Autozone) that if i want rear speakers to work, i have to buy a new aftermarket stereo, the adapter plate, the harness plug, and a bunch of speaker wire or RCAs depending on if i gonna run an amp. but i suspect that a trip to a junkyard, a knife, and some quality time with a Ford Windstar, and i'll figure out how to get rear sound on the factory radio. But it really sounds ok with just the door speakers, Just hate to let those speakers I already modified to fit behind the seats go to waste.