I've had this odd problem since I got the car. The taillights will work fine if the battery is fully charged but if it's not (just a little low), they wont work though the front ones still do. I ran a jumper wire from the rear harness to a wire that would run to the front side marker light but I'd like to solve the problem altogether. I replaced the rear light harness but that didn't solve it, my guess is it might be at the connector where the 20 gauge brown wire goes to 18. All the grounds are fine too, it's like there's resistance in the wiring that won't let the power get back there.