I repaired the connector plug housing on the seat switch today using 3M DP 270 black epoxy (the plastic on the seat switch housing did not respond to the plastic solvent cement). The broken plastic part had to be held in place with a vise. I cured the epoxy in an oven.
The seat lower side cover did have the plastic broken where the front torx socket screw holds it to the seat. This plastic seems to be styrene and the plastic cement worked well. It is now drying overnight, held together with a small vise.
If the plastic weld repair on the seat cover looks good tomorrow, I will bond a washer to it with epoxy. This should make it strong enough to last for a while.
I did take pictures of the repair process. I'll post them when I get a chance (please share some photo posting tips). I'll probably start a new thread about seat switch repair, so it will be easily found in a search.
Thanks for your help... you guys saved me hours of electrical troubleshooting. Hopefully I can contribute something back with these plastic repair techniques.
2000 E39 528i 5 Spd
Sport package - BMW Schwarz/Sand Leather - 6/00