There are several earth connections dotted about the engine bay, it may vary between engine versions too?
Just look for large gauge cables (usually black) with ring crimps attached to the chassis with M6/M8 bolts, quite often these have lost some of their conductivity due to various factors and can be seen to have rust or a white residue on them.
Removing these and thoroughly cleaning all surfaces (bolt included!) can work wonders for your tired electrics.
Its not an exact science though, more a process of elimination, car electrics are always going to be frustrating when they go wrong...
Sprint - 33 - Sprint - 33 - 155 - GTV - GTV - 156 - GT ...gone German for now though.