What needs to be earthed?

[Blas]

You could just use the wiring diagram from another manufacturer as a guide for the wiring and grounding. Consider making 1 or 2 specific grounding station lugs under the dash. One in the trunk and two under the hood. Remember to remove the chassis paint at the grounding points on the chassis and always add a star washer on the bolt connection. Always attach a large ground to one of the bolts on the starter. What you want to prevent is grounding thru the radiator and coolant. (Does not need to be as heavy a gage as power to starter because of short length of ground wire)
Buy a good tool for installing brass wire connectors with full insulating covers on the connectors, not those hardware store pinch connectors. Use shrink tubing at the connection of wire and connector. (all the same length) When harnesses are finalized, add non- shrink tubing or use self-vulcanizing electrical tape on the looms to prevent chafing. Rubber grommets where wire passes thru metal of fiberglass. Look into the different kinds of master kill switches. Using a knife switch as suggested, will not shut off a running car, and if used, can send a spike thru the cars electrical system when current flow changes from battery to alternator source. Do some research on the cut-off switches available, unless you only want a battery disconnect version.
Relays (with plug-in bases) should be used for all high power users (horn, fuel pump, fans, and headlights) and consider not using the single DIN127 single relay for low and height beam headlights. Split into two standard relays - one for low and one for high. Each fan should have a separate relay if you run two fans. And kick-up the wire gauge on the headlight wiring, you will get better results with illumination.
Blas