if its like that get another loom/fusebox i am 99% sure its the fusebox seen it loads of times once they burn up a bit the plastic goes hard and when you hit bumps it drops off

theres a fact that most car fires are wire related have a look at insurance claims/sites, you,ve seen the burning on a relay and once plastic does catch alite it self feeds itself even a small electric fire can right a car off
people may say i am wrong and say you can run a wire hear and there if you know what your doing thats fine if you make the old one reduntant but i carry my kids in my rs and if it means i have to replace a loom to make my car safe i will doit so my advice might be harsh but its the right thing todo from a cost point
dont get me wrong a new feed to lights and fan would be fine using your old fusebox but its damaged due to arking as you have said above so i would advise changing it just for the fact of above if its burnt like you say it aint never going to work right without a serious fix /bypass which will cost alot more than a good fusebox/loom if its sat on your drive for 3 months coz of the faults another few days work taking it slow and new good loom will be in, if you have had a few rs's apart it aint such a hard thing to do old out new in might look like it will be a handful but it aint
mark2