Need wiring diagram for Gardner Bender GMT319 meter

That particular meter does not have the ability to check OHMS

That is not something that you can do to specification. The resistance of that wire is actually the impedance. You are going to only be checking for continuity by setting a meter to the ohm setting and then either short one end against itself and check the other end or use both ends and put one probe on the collar and one on the center conductor. The meter should swing all the way to the right.

As with any single pole switch, there are two wires, or else two screw terminals. One is for the wire (or wires) that is always "hot", and the other is for the wire that goes out to the light fixture.

blew a fuse

there must be internal short somewhere - open unit up and check

Two LR44 button batteries.

Two ways to check. You can take them out and test them with the ohm meter . Or leave them in circuit and test 12V to ground on each side of fuse. Notice the two small metal tabs on each fuse. Touch your tester to these. You should get a reading on both sides. If only on one (incoming) and not the outgoing then fuse is blown. Note it will only work if the fuse is on (key on acc if it is a switched fuse)

The circuit breaker trips because you have some sort of short circuit. Use a Meter set to OHMS, and try to find the "Short". Check the wiring in the Plug also.

Google " gardner bender GVT-392 manual".... there is plenty of hits good luck

Set it to OHM or resistance and read the element