Lee, there is not a central grounding, or earthing point for the various electrical loads of the truck, most U.S. vehicles did not have this. Do/did U.K,spec vehicles do this? And, if they did, based on the history of Lucas Electrics, The Prince of Darkness, I would immediately rip my user-installed one out! LOL!

When I installed an apostate GM one-wire alternator (for which, by-the- way, I am utterly remorseful and contrite for having sinned and intend to eventually change to Ford. I live with this shame)and auxiliary fuse block for additional circuits for stereo, driving lights, loud horns, camper lights, etc. , I added my own central ground buss, and it has been nice. You might want to do that.

If you can, clean up any and all earths/ grounds that you can find, and you can even add a second redundant one for those accessories giving you problems. I had to do this for the NOS, period correct driving lights that kept giving me problems. After screwing around with their poor , undersized, and corroded ground wire, I cut it off, drilled the rivet, and installed a larger one back to by ground point instead of to the chassis. Problem solved.

I would not replace the TS switch till you have located and definitely corrected or re grounded all the bulb sockets and or light housings. I see you have done this. Replace, or add another of the main ground that ßpurredon spoke of in that other thread. At least, clean it up reall well. It could be the switch.