cab electrics fault on foden(late daf cab)?

Intermitten fault when various items like h/light flasher
heater blower etc used this interfears with tacho warning light and causes problems with as tronic g/box
Brand new box fitted but can,t trace wiring fault.g/box people blame truck.Had it on computer at foden dealers but they don,t seem really interested.I think maybe cab earth problem or loom under left of cab. any ideas? 05 reg