Originally Posted by g300d

Have you tried tracing the tail light socket leads to their plug pin outs to see if there are any differences per side? Also compare pin outs of the vehicle harness while you're at it.

You mentioned you replaced assemblies, can you compare the old tail light pin outs to the above?

I remember reading in the LED tail light threads that some people had to mess with switching positions on the harness plug to make their tails work right. Although you replaced standard tails with another set of standard tails, it may be the same situation in your case, especially with aftermarket units. Are your new tails OE or aftermarket?