It happens because of bacterial colonization within HVAC ducts. It is not uncommon. I have used Lysol spray to get rid of it. You need to turn your A/C on and set the ventilation to fresh air. Now step outside the car and find where the HVAC system sucks in fresh air. Spray Lysol on the vents for at least a minute. It will kill the bacteria that is causing the foul odor.
You may have to find a similar product if Lysol Disinfectant spray is not available in Pakistan.