I don't know what kind of heater you have. But start by verifying the wiring to the furnace. You can disconnect it at the furnace and check it with a meter or put a fuse in and see if it blows. This will at least verify that the wiring is good. If the wiring is good look at the furnace wiring for something obvious.. shorted wires or maybe rodent damage. Sorry I cant be much more help then that.
FWIW, a service writer recently told us that if we didn't have a screw running into a pipe or electrical bundle, we were lucky. If you detect a short between wires or from one wire to ground, you may want to check areas where the wiring runs close to the frame.
Fred and Carey
2014 Ram 3500 CC DRW 2wd (Max)
2015 V3605 (Valerie)
2003 H-D FLHP/2010 H-D FXDC