Bulbs blowing and NS tail light out

I hope you might be able to help me?

I have a 2002 K11 that I bought a few weeks ago. I love it, except it has revealed it has electrical issues.

I'll list what's happing, might jog a memory?
* OS headlight not working, changed bulb, OK so far​
* OS side light not working, changed bulb, OK so far​
* A blub in instrument cluster (LED odometer backlight) now not working
* NS tail light not working, changed bulb, did not help, still not working, all other lights in cluster working fine​
* I swapped the rear lighting cluster from OS to NS and the tail light still didn't work, all other light operated fine.
I concluded it's a broken wire somewhere, jiggled wires on display, no joy.​
* The OS tail light that works seems brighter than normal, might be my (and the AA guy that I called out)'s imagination though​
* Checked all relevant fuses near battery, one had the wrong fuse (15amp, should have been 10amp)
which suggests a bodge at some point to just keep the lights on probably.
I kept the 15amp in for now.​
Other minor issues which may or may not be related:
* Startup seems a bit sloppy/lacking in power. My other 3 older K11's (with distributors) jumped into life like a sprightly puppy, this is like waking an old dog,
Starts and runs ok, but doesn't seem to love the idea.​
* LED display in instrument cluster (LED odometer) is missing a few pixels until the vehicle is warming up, then they go grey. then display fine​
* LED pixels out completely on (factory) stereo, but display backlight is working​
* I have a cassette shaped adaptor that the previous owner included. It goes in the cassette slot and has an audio connector for plugging into your phone.
The first time I used it it was a really low audio signal, and had a background noise, like when audio equipment is not grounded correctly.
That is now working perfectly. Stereo also seems to work fine apart from inoperative LED display​
* When I got the car it had temp issues, I decided it was probably the thermostat. It was, it had been put in the wrong way round.
Fixed with new OEM thermostat, working perfectly now.​
I have the online and printed Haynes manual available. I thought to check/clean and grease cover all chassis ground points (10 relevant ones according to the Haynes lighting wiring diagram), but no idea where they are located? I found just one that looks corroded in the engine bay, and there is the main one to the battery, but not done anything to either yet. Not sure how the wiring loom is running through the car either.

I'm fairly novice at car repair, I do have a fair amount of regular tools for car jobs.

So, to fix the NS taillight, (and possibly other issues) where do you suggest I begin?