Check the ground connection from the battery to the body of the car. Also check for a ground strap from the engine to the body of the car. There may be more than one. It (they) will be flat braided cables if like a 740. Sorry I can't remember the location. Check the connection of the wires to the starter and the large cable attached to the back of the alternator (B+ contact).

Another Idea: Check to make sure the transmission is really in park or neutral. Try both. If the switch or transmission linkage is misadjusted, the transmission may be in gear, but the starter is engaging.

Good luck. You have found the right place if you intend to do your own maintenance.

Dan