You can start with the windshield cowl grill. Debris accumulated on the grill below the windshield tends to get in somehow and get sucked in the fresh air intake to the HVAC and eventually accumulate on the evaporator. Without removing the HVAC housing would be impossible to clean the evaporator so th next best thing to eliminate the odor is to purchase a evaporator core disinfectant spray. They come in small cans and has instructions you can follow to eventually eliminate the odor.