The main problem with Defender wiring, especially if it's an older vehicle, is that previous owners may have added stuff, taken stuff away and generally messed with wiring! I've had right grief with mine, and without electrical knowledge all you can do is fall bock on the basics and follow each wire back, through and along to where it ought to go. Get a wiring diagram, get a manual or two and try to work out what should be where and what is where ..
What's specific issues are you having?