Some notes on repairs to wiring and connectors, as opposed to replacing components that plug into the wiring harness. This is often necessary due to heat scorching wires, corrosion causing excessive current draw to melt wires from the inside out, cracked insulation from age, damage from oil or other fluids, wires frayed on other parts of the car due to poor mounting, routing, or restraint, and the like. Connector elements (pins, spades, etc) can get bent, corroded beyond repair, broken off, lose their tight fit, etc.
The most critical thing to remember when replacing wires or connectors is that great care must be taken to ensure that the repair is not creating opportunity for new corrosion or other damage! Otherwise you are merely postponing the return of a problem, instead of fixing a problem. Factory connectors and wiring are done with attention paid to minimize risks of corrosion. Repairs should be held to a similar standard. Obviously twisting wires together and wrapping in electrical tape is not sufficient.
Ideal is to splice in lengths of wire by soldering, and covering with shrink-tube insulation, unless an entire section of wire can be replaced from connector to connector without splice. A paint-on sort of electrical insulation can also be found, and is excellent also at sealing against moisture. Often replacement connector pins can be found at electronic and electrical supply places, look in the yellow pages. Make sure you have the correct crimping tools, as well. No-solder crimp-only type connectors are not preferred; at least solder those connectors after crimping to ensure longevity. Think about how moisture can enter the wiring, and take pains to ensure those paths of intrusion are sealed as well as possible.
As mentioned, damage to wiring can also take place because of where the wire is routed; when repairing wiring, make sure that wire is routed well away from any heat sources, is properly secured with tie wraps, is protected from rubbing on any metal that could cut through, and is never under any tension - cut wires longer than necessary, or splice in additional wire.
Any electrical issue found in our cars can be isolated, diagnosed, and repaired with a good basic understanding of how the electrical systems work, how they're depicted in the wiring diagrams, and how to check each part and type of circuit found in the car. A great deal of labor costs can be saved through some time "bonding" with the car, tracing circuits, and a lot of small, annoying, trivial inconveniences can be fixed permanently through some basic preventative measures. All of which means more time, money, and confidence to take your toy out and enjoy it the way it was meant to be - on the open road!