I had a similar sort of issue. Had just bought a golf, swapped the headunit for my one and noticed alot of the cables had been botched together, i just plugged mine in with the intention of sorting that mess at some point. Later on that day was parked up with my sidelights on, was out the car talking to people and suddenly someone tells me my car is on fire, i look into my car and the entire cabin was just full of white smoke, jumped inside, opened the bonnet, ran around and ripped my battery terminals off. Had the car towed home and then upon inspection the next day i found that the illumination wire, that connects the light switch to the headunit to let the headunit dim when you turn your lights on, had earthed itself and proceeded to melt all my headlight switch cables.
As you can see, absolutely melted everything together.
Ended up spending 3 hours unplugging as much of the this part of the loom as i could, that went through pulling all the cables away from each other as they'd melted together. Then with a sh!t load of electrical tape went around each individual cable insulating them. Finding breaks between cables and reattaching them. Then finally fixed the dodgy headunit wiring. 2 years on car is now with another owner and has had no issues with this.
