Are you sure it's not the little metal contacts on the bulb holder at fault?
Maybe try some gentle bending or swap a bulbholder from a working instrument light to check.
Also, check the little wires on the iirc capless type bulbs have not been bent out of position.
Unless the copper is really badly corroded (as in gone!) you could try cleaning - I've heard 'Cillit Bang' household cleaner is good on electrical contacts, also Coke (Coca Cola) or white vinegar.
Al.
