The most common place these critters tend to end up at is in the heater Blower motor, they gain access thru the lower windshield black trim panel that covers the wipers and then down thru the duct into the heater:

Solution! remove the trim panel and get a section of fiberglass screen door screen and cover the duct entrance and use duct tape to secure the shape as well as nylon tie straps to ensure it stays in place. I have removed more than a few heater blower motors and found nests within the "Squirrel" cage. If your blower motor seems to make a strange noise and or you get some bad smells it is a good bet you have a condo going on inside the heater!