First thing you should look at is the bentely wiring diagram for the starter, I have never replaced one but i would imagine there is three cables involved, a ground, a 12v++ (always hot) through a relay that is switched by the ignition switch. My best guess would be to look at the 12++ cable going to the relay or from the relay to the starter, with the wiring diagram in hand you should be able to find the right wire, NOTE: If you are not 100% sure pull the plus of the battery just for safety, do not underestimate the power of your battery its only 12 volt but packs a lot of amperage), you CAN NOT meassure resistance when voltage is on the wire you will fry your multimeter and possibly your hands. gain access to the wire somewhere and check the resitance towards ground (any metal part of the car body), you could even possibly do this from the fuse box actually. when you got your ohm meter hooked up try and wiggle the wires and see if you get a low/0 ohm through signal, once you KNOW its a short and i happened during the clutch work grab your mechanic by the balls and have him fix it post haste.
last step, find a new mechanic (if it was in fact his fault)
if all this measuring is unkwon territory for you have it checked out somewhere professional, again the electrical system is no place to be messing with when you dont know what you are doing.