Lots of possibilities. My favorite is to use the bulb from the 6AA model in a 4AA light that has been modified for straight series (it's easier than you think) and then run it on 4 nimh rechargeables. This makes a nice bright, white light and I just love those DB bodies. This set up works equally well on the 4AAA model ($5 at Wal-Mart) and I haven't had a bulb burn out in 5-6 battery changes. Do a search for Double Barrel and you'll find everything you need, except the bulb. But rechargeable may not be the best thing for the wifes car, what with the self discharge issue and, in northern Michigan, I bet you'll be concerned about cold weather performance.
For a light in the wife's car I think I have just the thing. I removed the plastic barrel liner/bushings and cut down a 4AA DB so that it holds only two lithium 123's in each barrel. I cut the barrel off at the head end and re-attached it with SS sheet metal screws. The stock series/parallel arrangement remains as is. I then installed a bulb from the 8AA DB model. In this set up, the 4 lithiums burn this bulb right at it's rated voltage of 5.5v It's not as bright as I expected but it's much brighter than the original 4AA, should have a good runtime (2-3hrs?) and will be great for cold weather/long term storage. I even re-attached the black rubber side panels using silicone adhesive. Looks like it was made that way. Did I mention that I love these lights?
