You really just have to use a powerful flashlight and look in any nook & cranny you can find or get access to. If you find the offender remove it and clean the area really well.
Next step is to try and plug up any holes size of your pinky finger or bigger where the rodents can get in. Steel wool is good for plugging holes where it's difficult to use wood or other material to cover it. Expanding foam won't stop them.
Other option is to find an industrial air freshener designed to mask these types of smells. Something like this http://www.scoe10x.com/ A little goes a long way.
Eventually the smell will go away on it's own (months or year)
