Kim, just a heads up.
It's something common to many old vehicles that have lain unused for many years and I'm assuming you haven't already started the clean up.
IF you're dealing with the bi-products of mice and rats,please make sure you're adequately protected, i.e. mask and gloves.
There's a few nasties associated with this,and disinfect the cab thoroughly after removing the aforesaid mess.
There's a thing called Hantavirus,it has been discussed many times here before, suffice to say you DO NOT want to get it!
Just lettin' ya know!