follow black wire from battery it should go to 1 bolt on body an then on to gearbox, the gearbox one is often the dodgy one
looking for the main one from alternator and thatThe HGT has a small one at the back of the cylinder head for the electronics. I'd bet the smaller ones have something similar.
Each light unit will have its own earth pint, but if lights are ok leave well alone.
mine only got power coming from altThe alternator is earthed through the main engine/chassis/battery earth. The wires are best part of 10mm dia so not too hard to spot