I use this site to find part numbers..
https://www.gmpartsdirect.com/?year...tor&p=catalog&mode=search&scat=&search_in=all
Have you taken the assembly apart to inspect the motors and wiring? It might be that the wires or connectors have corroded and can be revived with some soldering. Not an uncommon rust-belt issue. I had it happen on a vehicle once ..but only figured it out AFTER I went to replace with an expensive new heated mirror (not an H3).