By this I'm guessing you mean the starter solenoid won't actuate? That circuit provides the ground path to the coil side of the solenoid. First test the solenoid by shorting across the two terminals with a big screwdriver (do I have to say be careful? :-)).
Assuming that's fine, then yes, out with the wiring diagram and the multimeter and check continuity to ground on that circuit.
If that's not fine, fix the starter solenoid :-).
EDIT: I'm wrong there, that will test the starter, bypassing the solenoid. You can Hotwire your solenoid with test leads - connect the ground side of the solenoid coil to the frame ground and the positive side to the positive on the battery momentarily. If it clicks and spins the starter, solenoid is OK.
NOT THE BIG POSTS CARRYING THE CURRENT TO THE STARTER