If you have a multi-meter (or test lamp) plus a wiring diagram (in the Haynes Manual) then it should be possible to trace the fault.However the quickest (+easiest ?) method to get it working is to over-wire it...ie connect the power( via an in-line fuse)by wiring direct to the battery (or better still, a fuse in the fuse box)
RegWorts
May 2008