Wiring and the meaning of life
There will come a time when you need to do electrical work on your Little British Car, trust me, it will happen. Popped fuse, burned wire, unplugged connector, something is going to whoa you down.
If you wind up having to replace a wiring harness FEAR NOT oh intrepid British Car Person. You can do this. Honest.
There are two approaches. One is to tear out the old harness and install the new one from scratch. Assuming you have a replacement harness that's close to your model year then the color coding will work dandy. Red to red, blue to green, oh wait, is that right? Yep. Make notes as you go and use masking tape to mark the wires and connectors. THAT way when you put in the new harness it will help. Lay out the old harness near the new one and mark the new one with masking tape as you go along. You'll thank yourself for it later.
Approach number two is to install the new one as you remove the old one. Sounds easy but the old harness does get in the way as you go along.
I subscribe to approach number one. It's cleaner. Course I've done harnesses a time or six so that helps.
Remember, planning is everything. Go into the project with your head held high and knowing that you can do this. It just ain't that big a deal.
Here's a generic wiring diagram for you.