Have you tried the two main connectors that go into the bottom of the fusebox either side
Check for corrosion and condition.
One is straight from the battery, the other from the alternator.
The connector of one of mine broke when i was out driving and it resulted in me loseing half the electrics.
Also worth removing the fuses, and giving the contacts and fuses a little clean.
Sound advice, would only add to that/suggest that it's worthwhile taking the fuse box and relays out
completely for a damn good clean up. Easier out of the car than in
Worth checking the principal earthing points at the same time, a wee bit of corrosion can make a startling difference to the electrics.
Ahhhhhh..... the memories