[rotorwrench]

Disconnect all circuits from the known buss then reattach each one by one to see if one blows the fuse.

The model A has a simple system compared to later cars. A person can check each wire to ground with an ohms meter to see if there is a dead short. Wires that are close together can be checked for cross shorting also. A person can check a system out with no connection to the battery if they do it this way. Individual wires should be isolated to do the checks.

I use my multi meter a lot and I have lots of jumper leads of all different lengths to check out individual conductors. The lighting system is the most common culprit due to the amount of current conducted and the amount of separate lighting circuits. It's more simple if you only have one tail lamp since there are only two wires that follow the frame back to it. Headlamp wiring runs inside metal conduits and along the radiator so there are multiple places they can short plus the horn wire that is hot all the time.