Running the wires under the hood
A new harness is one of the best things you can put in an older car, and the way it is routed decides whether it stays trouble free. The wire itself rarely fails. Chafe points, poor earths and heat do the damage, and all three are decided in the hour you spend planning the route.
Plan the route before you fix anything
Lay the harness loosely in the bay and find the natural path first. Follow the original route if the car still has its clips and brackets, because the factory route was chosen to keep the wires away from the exhaust and the moving parts. Where the original route has been lost, work from the bulkhead outward, keeping the harness high, along the inner wing, and dropping to each component with the shortest branch that still has slack. Slack matters at anything that moves: the engine on its mounts, the bonnet, the throttle linkage, the steering.
Heat and chafe
The exhaust manifold is the hottest thing in the bay and the most common reason for a melted loom. Keep wiring a hand's width clear of it where you can, and where you cannot, sleeve the branch in heat resistant tubing and clip it so it cannot sag. Every hole a wire passes through needs a grommet, including the small ones through brackets, and any wire that crosses a sharp edge needs either a grommet or a fixed clip either side so it cannot saw.
Earths decide everything
Most mysterious electrical faults in old cars are earth faults. An earth needs bare metal, not paint or primer, and a star washer under the terminal. Give the engine a heavy earth strap to the body and another to the chassis, earth the lights and the gauges to the points the wiring diagram specifies rather than the nearest bolt, and clean every earth point back to metal before assembly. Take a photograph of each one, because an earth added later under a bracket is the hardest fault to find.
Loom, clips and looks
Original looms were wrapped in cloth tape or a woven sleeve, and modern equivalents are available if you want the bay to look right. Corrugated plastic conduit is practical but hides chafe, so if you use it, keep the runs short and inspect the ends. Support the harness every hand's length with P-clips, rubber lined where the clip touches the loom, and use the original bracket holes rather than drilling new ones. Route the harness so a branch can be unplugged without disturbing the rest.
Relays, fuses and extras
Headlamps, fans and fuel pumps are worth relaying, and relays belong somewhere dry, high and reachable, with the fuse close to the supply rather than at the far end. Accessories such as an anti run-on valve, an electric fan or an ignition module each need a properly switched feed and their own good earth; check which side of the ignition switch they belong on in the wiring diagram for your car rather than borrowing a feed that looks convenient. Every added circuit gets its own fuse.
Label, test, then tidy
Label both ends of every wire as you fit it. Test each circuit with the harness loose, before the clips go on and before the trim goes back, because that is when a mistake costs minutes instead of an afternoon. Tidy last. A harness that is neat but untested is only neat.