Mold or mildew has undoubtedly moved into your HVAC air ducts. Turn the vent on and the fan on high, then spray the air intake with a large amount of Lysol (from an aerosol can) and then immediately shut the fan and vent off so the Lysol can stay inside and work to kill off the mold or mildew. The fan will suck the Lysol into the interior of the HVAC ducting. The air intake is underneath the cowling just in front of the windshield. Don't turn the vent and fan on for a few days. Then I would do it again after 3-4 days and perhaps one more time 3-4 days later. It takes some doing to kill all that off so one application is seldom enough to actually kill what is producing the odor.
P.S. Don't bother trying an odor masking/neutralizing product like Febreeze. You have to use something like Lysol that will actually kill the bug that is causing the odor. Bleach would work too but I hate the thought of spraying bleach anywhere.