Originally Posted by SQLGuy

The three position switch for selecting memory is shared by both the seat controller and the mirror controller, and doesn't distinguish left or right. Since all three positions work correctly for the seat and for the right mirror, I don't think this could be the area of the problem.

As for the under-seat unit, the only connections between it and the mirror control module are the 1/2/3 signals from the memory switch, power, and ground. Again, nothing that differentiates left from right behavior.

Cheers,
Paul