Its a Loom mate, you could use a multi meter and then disconnect each wire in turn and check the continuity but this would take some time.
If everything was working before then there is a good chance the welding has melted the loom.
I would just replace it from the plug in the engine bay to the junction in the rear corner.
You can pull the replacement through the chassis with the old loom section or run it external on top of the chassis rail.
A length of heavy duty trailer cable is good for this job.