Probably mice are in the heating system.   I found at least two dead mice when I pulled my dash.I soaked the heater tubes in simple green and in vinegar as well - worked well.  One painful place was the hollow "Knee tube" across the front of the van that ventilation is fed into.I duct taped a garden hose piece onto my garden hose and vacuum out this space.  Took many tries, because the hose gets blocked, but you need a small hose to get in there effectively.In the center of this bar, where the heater box mounts to, there are fins, and those are a pain to clean.
A  lot of work but there is no mice smell in my van and I have 100% peppermint oil in the van now through winter!JohnLiMBO  www.limbobus.org