Any testing at starter motor, make sure tranny is in park or neutral and parking brake is set. Usually two voltage circuits at the starter, one comes from the battery, hot all the time. The other circuit goes hot with key in the crank position, it may be wired through clutch or neutral switch, depending on the tranny. It may have a starter relay? The starter motor usually grounds through the engine block. I don't know if you have a security system that can effect your starter motor?
I need year--make--model?