I had to do this two years ago on an Opal that had 12v electrics.

As John says, they popped up with a fair shunt, keep them on longer than 20-30 seconds and they start to smell very hot.

I tried a 12v to 6v dropper but they barely came up, in the end I fitted extra bulbs in line to them under the dash - wasn't perfect but stopped them overheating.

A resistor in series and equal to the resistance of the solenoid is the right way to go. You will initially get the full 12V across the solenoid when you first switch on which will quickly drop to 6V as the current increases so it should be a bit snappier coming up than when on only 6V supply.

Location: Royston Hertfordshire

Nick,
I was of the impression that 12v units are quite readily available, buy some and donate the 6v ones to me. Well you have to try don't you

Location: Oakley , hants