I've never had a mouldy car.
Start by giving it a good clean inside, out, and glass - mats out, seats slid or folded, hoover first including boot and the grooves in the doorseals. Sponge all interior plastic (handles, dash, wheel, controls) with warm water and WUL, and polish off with an old vest.
Then, when you can seee the mouldy bits, kitchen cleaner with bleach spray will clean it off. Rinse of with water and polish off with that vest.
Mats can often be hosed or sloshed about in the sink then hung on washing like or fence to dry. They will be surprisingly dirty.
If it is mouldy inside, you probably have a water leak. Most likely round an opening window. Unless you drive around steamy, with the windows shut and the heater on "recirculate". Feel round for wet carpet patches.
If you have air-con, it will dry out dampness.
It will take some time but be satisfying. If you are in a hurry, just do the inside.