maybe u can take out the motor and jump it to see if it will run out of the car? that will rule out the motor itself. also hook a multimeter to th wires and see how many if any volts u get when u push a button.

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05