A 6v zener diode capable of handling the current. I REALLY don't know what THAT is - I've never heard of it, but I'm not experience with electronics in any stretch of the imagination...
Basically, I'm shopping (on eBay) and I'm looking for "period correct" 1949 Chevy *Guide* fog lights - and I need to know if I see a really nice pair , but they're 6 volt , can I still buy them and somehow use them on my 12 volt truck?