When the new holder and bulb arrives I suspect I would attach it to the wires and hopefully see it flash as it should if that is the cause of the issue..?
I would be tempted to try similar now. With a short piece of cable with bared ends, push one end doubled over into female bullet with blue cable and carefully hold other end to metal body of a bulb known to be good. Then with o/s indicators selected and flashing elsewhere, very carefully touch the bare bullet with brown lead onto centre solder blob of bulb, avoiding any short.
With luck it will flash but given the cleaning of bulb holder and connections already performed by Peter, I have a suspicion it might not. Hope I'm wrong though!
