Oldsmobile/Buick Repair: 1996 Oldsmobile 88, oldsmobile 88, ventilation system

Question
I just inherited a 1996 Olds 88 that was unused for about 1 year. Mechanically everything appears fine and the car runs well. The problem is a horrible smell that comes out of the ventilation system when it is on either A/C or heat. Are there filters to change or any way I can clean and sanitize the inside of the ducts?  

Answer
If the horrible smell is antifreeze, the heater core may have leaked.

if it is mold or mildew, you might need to get into it...not easy...and wash it.

Then...there is the problem of mice, or other rodents getting in there and dying, or just soiling in there.

No filters.

Van