halbro,

Check the fuse first (if I am not mistaking, the heated seats are separately fused), than take out the wooden panel with switch and check ground and power at the pins in the connector. If these pins are powered and grounded you have to try to exchange the switches to try whether this one malfunctions (these switches are easy to take apart if you want to check the inside).
You can check the cable harness with a ohm-/voltmeter on both connectors (switch-side and seat-side).
If the electrical system works alright so far you have to check (possible) power at the connector underneath the seat.

greetingz & good luck,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)