Stewart....the rust you have is quite nasty. The lamps are noted for leaking rainwater in and many fill up. The neoprene seal under the lens can deteriorate. Also, the rear that sticks back through the wing is in a VERY expose location with water from the tyres being thrown over it.
Solution many do is drill a small drain hole in the bottom of the lens, use silicon sealant under the wing. I sprayed Dinitrol sealer over mine.
Replacement bulb holders are not that expensive either.
