Most 12V wiring projects are well within the ability of the average DIYer, but you can get good results and operate appliances safely afterwards by doing things properly. Most problems are due to poor connections, badly selected components and poor circuit design. Bear these tips in mind when you attempt any 12V DIY.
NB If you doubt your ability to do any electrical work, have the job done by a qualified professional.

Crimping terminals
You will probably be crimping terminals if you do any wiring. Buy the right crimping pliers for the job, ensure you remove enough insulation for a good connection, and if the terminal doesn't crimp properly, use a new one instead.

Battery connections
Getting power to an accessory depends on having a good battery connector. If your battery terminal is old and bettered, buy a new one. When making new connections, use an appropriate terminal for the current it will be handling.

Wire rating
If you aren’t sure what the power requirements of an accessory are, it’s always better to use a heavier wire to be on the safe side. For 12V work, 17A-rated cable is the heaviest you should ever need.

Fuse considerations
Every accessory that is connected to the battery should be fused. The fuse should be placed as close to the battery box as possible, but not in it – if the fuse blows, it may give off sparks and ignite vapour from a charging battery. Check the accessory instructions for the appropriate fuse rating.