Check relays and inspect wiring
Unless things have changed greatly the most possible answer is a lack of earthing, cars etc electrics mainly rely on a positive feed via cables with a return through its chassis or body.
When this fails all manner of weird effects can happen because this positive feed unable to return in a normal way tries other less direct routes via other circuits and creates problems.
I would examine that defective light cluster for bad contacts from those bulbs metal case to earth possibly a black wire to a chassis screw fixing.
Another fun place is some light clusters have a metal reflector or conductive layer which acts as a link to this earthing, corrosion can exist at screw fixings or in bayonet fixed lamps they can have folded or crimped sockets to metal/reflector which corrosion can occur all around this point, from condensation forming then rusting this area.
I would after checking take a long wire with one end fixed to the chassis as an earth touch its other bared end to your defective lamps metal case (or ground lead) to see if it works OK (Not its centre or live pin).
If it works then look for a bad earth connection, if not then you will need to use a voltmeter to confirm that power is getting to the bulbs centre pin or positive feed.
In most cases I have endured it will most always leads back to earthing issues in older cars.
Hope this helps.........
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