you know the little packs of silica that come with computer / electronic products to keep them dry, well homebase sell a large version of that which comes in a plastic box with a shelf for the silica packet and a tray to catch the drips.
my car used to p1ss water off the roof and back window and i'd always have soggy carpets in thge boot and in the back footwells. i've sat one of these in one of the back footwells and it's working really well, both windows and carpets are dry