Replace the cabin air filter, which is located under the windshield cowl on the passenger side. You should also spray some a/c vent disinfectant in there before installing the new filter. You can purchase this at almost any autoparts store. Mold can form inside the air ducts and cause them to smell with the heat. The disinfecting spray will kill it and freshen up the air. Also, mice like to nest right in front of the cabin filter so I would guess that is the source of your stank.
