I'm in the middle of diagnosing my heated seat problem. I can only get the driver's side to heat on Stage I, not Stage II. Passenger side used to work, but now doesn't heat on either setting. I just recently read through the shop manual information.
Check the wiring under the seat. Make sure connections are tight, and that there's continuity in the heating elements. After everything checks out, the problem may be a bad module.
95 E320 Cabriolet, 131K