Sounds to me like a short or bad earth somewhere. PITA as it seems I'd begin by tracking down all the earth points for the wiring and make sure they're ok. If that doesn't show anything obvious up then trace wiring loom in general and check for chafing/bare wires that might be shorting out and causing the faults. Either way it's going to be a long, tedious job
Is there another owner living near you that you know who'd be willing to help out? Jobs like this always go better/quicker with a friendly helping hand and some beers afterwards of course 
Good luck whatever the case though! 
Cheers,
Mark