A multimeter is a good place to start, get one that buzzes when reading resistance
I kind of do this.
1.. Work out which circuit (ie components) is supplied by which fuse
2.. Disconnect battery and remove fuse
3.. undo fuse box and remove feed to fuse and wire or wires that go of how knows where!
4.. Use a multimeter on Ohms (resistance) with one lead on earth (ground) and the other on the wire - shows low resistance?
4a. If yes then unplug the component or components
4b. Still yes then there is a wire problem
4c. No then it's the component or components
4d. Plug them in one at a time until the meter show low resistance
OK it's a pain to get at some components to plug and un plug them, but have you changed anything recently, fiddled with stuff?
5.. Remove suspect component and if possible test with flying leads and a battery - be careful!
Usually wires are ok until they start to move around and or start to "rot" internally, usually go black.
It's a pain to do but work methodically through the relevant circuit and all components including switches etc. Look for evidence of melting wires too
Malc
