The seat heaters were standard on the 500 SEL and the switches were located right where the console meets the dash. You should have a small storage well right below your ash tray since you don't have seat heaters. If you did, this area would have an elevation and the switches found there. The rear seat heater switches are found on the rear door where the window switch is located.

I would love to have had the heated seats on my 300 SD but I've never seen one that opted for them.

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1984 300 SD
1977 450 SL Euro