Mike B is right.
Check carefully the following 4 interior locations (poke with a wood dowel - with permission of the owner:
- lower rear cab corners - near the “weep holes” (rodents like to nest/[censored] there) and moisture absorbing degree collects there
This is an expensive, time consuming repair.
- lower front cab corners - where the side walls attach to the front wall and the floor board
Good luck.
