There is a plug-in diode in the bottom of fuse box that keeps the switched circuit(key-on) from receiving power from the battery power-on bus for the electric seats. If the diode shorts out this will happen.

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MERCEDES BENZ MASTER GUILD TECHNICIAN (6 TIMES)
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. Shopforeman
190E 2.3 ITS RACECAR
1986 190E 16V