OK... As promised, here is the description of my procedure to fix the seat switch:
The switch I am repairing is the driver's side seat switch on a 6/00 built 528i. It seems they all break the same way.
An older thread explains how to remove the plastic side cover on the seat. Usually this cover is partially broken, which pulls on the connector to the seat switch and breaks the socket housing.
After removing the seat switch: Clean the seat switch overall, and then carefully clean the cracked plastic where it will be bonded together. I used 99% isopropyl alcohol for this.
The seat switch plastic will not respond to plastic cement. I used 3M DP 270 epoxy, which is meant for potting electronic components. DP 270 is black, very liquid when uncured (compared to most epoxies), and cures very slowly. It comes in Duo-Pak cartriges, and a "gun" is required to us the cartridges. The gun is pricey but made in Switzerland and a lifetime purchase. I bought mine nearly 10 years ago.
I'm sure there are other products that will work, but I will describe the DP 270 procedure since this stuff seems to be the best material for the job.
To avoid the waste of all the epoxy in a mixing nozzle, I squeeze the trigger on the "gun" and create a bead of the two components side by side on a old business card. About a one-inch bead will do. Mix the two components together for 60 seconds.
I apply the DP 270 with a cheap plastic paintbrush from the hobby store. It goes on like really thick black paint.
Since the connector socket housing had distorted, the broken piece would not stay in place by itself. So after putting DP 270 over all the mating surfaces, I then put the parts together and then used a very small machining vise to hold it together. Some Boeshield T-9 oil on the vise will prevent the epoxy from bonding to it. Excess DP 270 gets brushed along the sides of the connector housing like paint to provide extra strength.
At room temperature, DP 270 takes 48 hours to cure. I placed the part (with vise attached) into an oven at 150ºF for 90 minutes, plus a slow cool-down. After that heat soak, the epoxy is fully cured. Cured DP 270 feels like hard plastic, not the gummy consistency of many epoxies.
The plastic side cover for the seat had the front hole (for the Torx T-30 screw) cracked. This plastic responds to plastic model cement, so I used that to bond the cracked area together (and also any other splits/cracks in the plastic). The cracked hole was pressed together with the same small vise and allowed to dry overnight. Then I bonded a washer to the outside of the hole for the T30 screw with a fast cure epoxy.
Put it all back in place and it works perfectly. Can you imagine how much the Stealer would want for a repair like this?
Thanks to this forum for saving me hours of electrical troubleshooting by sending me straight to the cause of the fault. Hopefully this DIY will be found useful.
Photos, left to right:
1. Broken seat switch with broken-off part
2. Seat switch with uncured epoxy and vise holding it together
3&4. Finished repair