Yep, you've isolated the circuit that's overloading. Now you need to determine if you've got a dead short, or just too much current draw for the fuse. Try this- - - -make up a jumper lead with an inline fuse in it. You can get an accessory fuse holder at any auto parts store, and attach an alligator clip to both wires. Splice extender wires to the fuse holder leads if necessaary. Put a big fuse, maybe a 20 or 30 amp, into the holder. Attach one end of the jumper to a known source of voltage like the battery terminal on the ignition switch, for instance, and touch the other clip to the wire you just disconnected. A big flash and a blown fuse confirms that you've got a dead short, a hot lead attached directly to ground. If the lights burn normally when you connect the jumper, and the wires don't get hot, your circuit is simply drawing more current than the fuse you selected for the light switch can handle. Step up in fuse size until you've got a little bit of overload capacity- - - -if a 4 amp fuse blows, try a 6 amp, or maybe a 10 amp. Just don't get ridiculous and go with a 20 or something similar. The fuse protects the wire, not the end item, so if the circuit you're powering has a #16 wire, for instance, use a 10 amp fuse or smaller. #18 wires need about a 7 amp, and really small wires like a #22 or so should be fused to 5 amps or less.
If your dash lights are drawing more than 6 or 7 amps, you've probably got a short somewhere.
Jerry
