How to find and wire in an analog balance control

I have a special need to add an analog balance control - a knob that I can easily turn while driving. I need to be able to pan left, center, or right without killing my self trying to manipulate by radio's digital menu and drive at the same time.

I sure I can buy something to do that at Radio Shack. What shoud I ask them for and how would I wire it into the speaker wires coming out of my radio?

I have a 2000 Jeep Wrangler with an after market radio and the sound bar (I have four speakers, two in the front and two in the rear in the sound bar). I don't mind drilling a hole somewhere to mount the control.

As to what my special need is, I sing barbershop (four part) harmony with a chorus and in quartets. The way we learn our songs is with special learning tapes (mp3s, actually) that put one of the four parts on the left track and the other three parts on the right track. When learning a song, I start by listening to my part (baritone) by itself by panning the tape hard left. As I get to know it, I pan center to begin singing with the other parts, then eventually pan hard right to sing with the other three parts alone. Since I live and commute in Los Angeles, the best place to do this is in my car. So I need to be able to easily pan left and right without taking my eyes off the road.

Thanks in advance for all advise.