TL Stereo Question

First tune to a mono AM station to ensure the same signal on both Left and Right. Fade to Front and use the balance to check for side to side equality Fade to Rears and use the balance to check for side to side equality

If you experience the same imbalance on the fronts and rears the speakers are not LIKELY the problem. To isolate, swap speakers from right to left and repeat the tests. To isolate further you'll need a substitute amp. Try a wrecker for a cheap used one, repeat test, if it does not work take it back and pick up the head unit. Also check for wiring problem between the amp and head unit.