Marine 12-V lamps have two nipples. The "shell" is isolated from the filament, and the connection to the filament is thru the two nipples.
If one of these is replaced with an automotive, dual filament lamp, it will cause exactly this problem.
Perhaps they installed marine lamps in this Palomino?
2005 Georgetown 342DS XL, Full Banks Package
2005 Liberty Renegade 4X Toad
1989 Wrangler Sahara Toad
1984 19.5' Beachcraft i/o 5.o liter, 4-bbl (and you thought your Moho sucked gas!)
D/W Carole and Maggie the Teacup Lab (Black Chihuahua)