I'd be betting it is the stuffing from your seat or from the padded insulation/sound deadner from above the headliner or from the front wall. Mice like to use it for bedding. It could also be from some stuffed chairs that you had in the garage and the mice moved it there to build a nest. I've had that happen to me a few times before. Somehow they managed to pull the stuffing out of a patio deck chair and build a nest in behind the shroud on my garden tractor.