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).
If you can paint it, paint the drawers to remove musty odors. There is a product called Niloder on the market. Available from some antique supply houses. Or you could remove and bury the dead cat.
Buy a new one
dryer sheets in the heater vents may help
Musty, dusty, humid.
Put an open box of baking powder in it.
Vinegar
you wash it????
It is most likely from mold in and/or under your upholstery.
Unplug clogged evaporator drain plug.
stick an air freshener in the interior vent
Go to a professional cleaning supplier and ask them. They have the knowledge and the products to do the job.
"The musty smell of the attic filled my nostrils" BWHAHAHAHAHAHAH
The first step to getting a musty smell out of a closed up house is to open the windows. You will also want to burn some candles. Spraying Lysol will also help get rid of the musty smell.
Bleach
There is a product called Zero Odor that when sprayed in the car will remove the urine smell.
Kerosene smell car carpet
wash everything and febreze...