If mold has gotten into the wood or lining of your trunk.
Here's how we usually deal with it:
Use plenty of ventilation in this process to avoid brain damage.
1. Strip the trunk of its lining. They are usually lined with paper or cloth. Sometimes this will almost fall off in your hands. Other times it is very difficult to remove, especially in certain cases where cardboard is used in the lid or base of the trunk and glued in.
2. After every trace of lining is removed, wash out the inside of the trunk with a 5% bleach/water solution. Rinse with warm water. Dry it with towels. Let it sit until it is completely dry. A little time in the sun won't hurt at this point as sunshine is one of the best things for killing mold, but watch for warping.
3. Now sand down the inside with 80 grit sandpaper and vacuum it out.
4. Next, apply at least two coats of oil based polyurethane, letting it dry between coats. You are now ready to reline your trunk and will not be bothered again by that musty odor.