There is a simple test to check if the problem lies with the starter itself or the control circuitry. I don't remember all the details, but it involves the three pin connector located on the firewall just above the brake fluid reservoir. One of the three wires from this connector carries a "starter engage" signal to the starter solenoid. The test involves connecting a voltmeter, then turning the key to check for the presence of the signal. Again, I don't remember details or specifics, but I know this has been presented in the past by Mr. Dalton or MB DOC.......

good luck

J. M. van Swaay