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90% of the time they enter through the fresh air intake opening underneath the dash. I had a customer that three times came in with nest built in the blower motor after they chewed through the in cabin air filter. About the only way to stop them is remove the blower box assembly and put a piece of fine mesh chickenwire or similar) wire mesh between the opening of the box and the sheet metal body and bolt the box back in place. Unless my super rodents have learned how to chew through wire this stopped the particular problem with this one customer.