Don't delay and get a mousetrap baited w/ peanut butter inside the trunk pronto!.
Keep checking it regularly and resetting it as req'd..
If the mouse piss smell has already happened, while cleaning the interior make sure to drop the underdash heater/defroster duct hoses and clean them out too..
My chum had an invasion of mice last winter in his car carrier and is still cleaning and fighting the smell..
Searching online he's read some horror stories about how hard the smell is to get rid of once it permeates the interior and how it attracts more of Mickey's pals..
Also someone told him putting old Cat$hit in the vehicle helps keep them away and smells better than mothballs?.
As far as the smell in buddy's truck goes, like the Seinfeld episode w/ the valet parking guy, the smell has taken over and he's cleaned and recleaned everything but it's having no lasting effect..
Next step is he's gonna start a couple small logs on fire in a steel bucket and smoke the interior out w/ the windows closed...all while standing by w/ a fire extinguisher of course..
Good luck!.
~ Pete