2007 E350 Mouse Smell in Heater

The reason you don't smell mouse when the AC is on is because it's cold and doesn't atomize the mouse particles. Heat & AC share the same ductwork. I'd begin with the more accessible areas, the engine compartment and at the duct venting, and move to the dash area. You'll then come to the areas where the dash has to be disassembled to reach it. It might be better for you to just let the rotting baby mice decay than to tear apart the dash, it's really difficult and can be quite costly. There are aerosol cleaners & deodorizers you can use, along with white vinegar, to palliate the smell, and after a few months you might not notice it anymore, however others might.

There's flexible telescopic cameras and grab tools if you want to do an inspection of some kind , Home labyrinth may rent the camera . A ford dash can be comparatively easy to pull apart as I remember