I've had it happen to me as well, and don't use Maglites for that purpose anymore.

But I think this is more about the batteries than the light material. I think alkalines self discharge fairly fast in this type of environment, and as I understand it, discharged batteries are prone to leakage. Then the leakage toasts the light.

I've had several different lithium based lights in the car and none of them have corroded, including aluminum ones.

Some other things to consider:

1) Even in a plastic light, if a battery dies and spews it's guts in there, it's going to be a major pain to clean out and in general, my guess is you'll probably throw it away anyway.

2) The plastic lights still have metal in them for the switches and to conduct the power, so you can't get away from it.

In general, I'd look for a lithium light like the Surefire G2, toss it in there and be happy.

Also note that you can get lithium batteries in the AA (and now, even in the AAA) size.

-john