[Hanuman]

Flasher relay might be behind the instruments. Have a vague recollection of it being out of the way. Or is that the hazard relay. Either way, one of them is somewhere stoopid.

Earths?

You might have issues measuring straight voltage at the socket, as some DMM meters simply won't respond sufficiently fast enough to give a proper reading, whereas doing a continuity check will give a more clear result. Check supply pin of the dodgy lamp to supply pin of one that works. Old wiring, so don't expect zero ohms, but you should be close.

Is there an alarm system in there anywhere?
I know I bang on about them, but the damage done with skotchloks is worth grumbling about.

Welded joints in the harness don't often fail, but chafed wires will be a real possibility.

In the lamp itself, don't discount a cracked contact in the housing. You might get a supply to the pin, but once a bulb is in place, it puts sufficient tension on the contact that it may open the crack. (Ooh er)
That had me cursing a bit on the V8!