Heater problem in '99 Jeep Cherokee-output blocked?

It appears that a squirrel has stuffed acorns into my Jeep and now, when I try to turn up the heater fan, a horrible crunching sound starts and little bits of acorns are spewed onto the penger side floor. The heater itself seems to work, at least if I've been driving steadily (highway) for a while, but the fan doesn't work at levels 1-3 and at level 4, all hell breaks loose. When I open the hood, nothing is obviously wrong, so where would I look to locate the blockage, and then, how would I go about cleaning it out? All responses are greatly appreciated.