Okay so maybe I'm the only one to have had this problem so far??
Well at the risk of talking to myself (but for the benefit of future searchers), I discovered the source of the problem. It was the headlight switch in the steering column.
There are no headlight relays, beyond the red/brown one in the engine bay. Instead, the full current for the high and low beam are switched in the steering column. Sounds dodgy to me, but to be fair the switch contacts look just like relay contacts. So, I guess they do the job - until they build up a coating of oxide after 30 years of switching.
The fix was to remove the 'combo' switch (it also does the indicators), prise off the cover and clean up the contacts. I did this in-situ to avoid unpinning the switch from the wiring harness. Once you get the steering wheel off, the combo switch is on the backside of the steering column switch assembly (along with the wiper/wash, headlights, cruise etc). So, you have to unscrew the whole switch assembly to get at it - its only 4 screws though. 4 more screws releases the combo switch. The combo switch has two inner compartments - the indicator side is full of conductive grease, while the headlight side is dry. As you take the lid off the combo, carefully remove the indicator switch and its contact rail and put them to one side. This saves them falling in the dust while working on the switch. The headlight contacts are fixed firmly in place.
This guy describes how he did this job on his 4Runner, which gives you the idea. Instead of a file, I used 10mm strips of wet & dry, folded in half lengthwise. I used 600 followed by 1200 grit, then sprayed it with contact cleaner to wash away the filings. A quick check ensured that it was in fact working before reassembling everything. Don't forget the indicator switch components!. Reattaching the combo switch, the switch assembly, steering wheel, covers, etc. is just the reverse process. Good as new :-)
Maybe this will help someone else who faces this issue in the future ...
Cheers,
Adam
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