Most auto parts stores, ie autozone, advanced auto, sell a/c deodorizer. It's caused by ciondensation in the a/c ducts. This is pretty common.
change the cabin filters
I found this tutorial "How to Remove Odors from Your Car" that may be of benefit - see relevant link below .
Burning Wires.
Depends on what the smell is? Can be many things including mildew or mold in the HVAC system.
It can be a sign of mildew or mold.
Get a new one.
the best possible guess I could think of is mice... If it's a chemical smell, you might have a heater core leak. If it's a mildew smell, maybe you have an interior air filter that is funky.
you must find the mildew or mold source first- you can't get rid of the smell if you don't know where it is coming from- if it's drywall i have inserted a ehow link- http://www.ehow.com/how_2056875_eliminate-mold-from-drywall.html you may need prfeesional help if you cannot locate it yourself- check for water damage and go from there Suggest you find where the mildew is located and remove it. One place to check is the A/C drip pan and the A/C ducts. If in A/C Ducts, need to clean or replace them. Also, look on line for mold and mildew remediation techniques.
could possibly have a rodent or rodent nest inside the heater box.
If you have air fresheners it might help. But if the smell is coming from the seats you might need to replace the fabric on the seats. It just depends on where the smell is coming from. Also worth trying are Febreeze spray on the carpets and fabric (it may require repeated applications) and/or Bounce dryer sheets under the seats. Depending on where and what the smell is, shampooing the carpets and upholstery may be effective. Try to keep the interior of the car as dry as possible, musty smells could be mold or mildew which love moisture. Repair any leaks that can be found (trunks are horrible for getting wet and breeding molds and mildew, check yours if you have one).
The holes allow the smell to escape and so the clothes will not mildew.
Mildew or other fungus growing underneath it.