bad wiring

Anyone got any advice on how to get the dark coloured corrosion of the surface of the cooper wiring in the vehicles loom. I decided to clean up the loom in the rear of the vehicle but solder will not stick to dirty cooper. I have ended up trimming it back down the wire until i reach the clean stuff but i am sure somone knows a trick to get it clean

Acid flux. Be very careful.

is that the stuff they use for cleaning off welding flux and other dirt after welding? almost a jelly like substance?

No

aka Baker's fluid. But you have to rinse it off in water afterwards or it'll corrode the joint away. I scrape off the oxide with a sharp blade until there's enough shiny metal showing and hope the resin flux will take care of the rest - I've got a small stash of olde worlde multicore solder for working on "legacy"kit.
If I can't make a decent joint I'll cut back to clean copper and add another connector and extend the wiring back to its original length.