On my '74 (which leaks like a sieve -- don't they all ;-) I carry around a tub of stuff called DAMP-OUT ... it's some kind of gnarly-ass salt, like you get in those little packets comes with stereos and whatever when you buy them ... that say "Silica Dessicant Gel Do Not Eat".
Bought it at the local Ace, I think. It sucks all the moisture up, turns into slush; I just pour the excess water into the garbage, keeping the goo in the tub (it's like a big plastic butter tub).
This last wet season the tub overflowed (musta been a LOT of moisture in the truck, LOL) so I put it in an old litterbox I had handy to catch the overflow (what can I say, I have six cats ;-)