Thanks very much for the tips.
The electrical system has been improved, relays in place of all the high-draw items that used to go through the ignition.
I know my way around a DMM pretty well, I'll do some poking around today and see where voltage starts and voltage stops, and also check the fuses.
When I asked Roger about the passenger window not operating, he said to give all the fuses a spin, as they tended to corrode over time. The window thing I believe was just me not pushing on the button correctly.
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