Somehow a critter, likely a field mouse, has crawled into my '77 cab ..... and has died there. It's small, but it's for sure there somewhere. On the driver side kick panel vent there is a sliding door and a screen of sorts as I recall. My truck originally had heater and defrost .... no dash vents as was no AC On the passenger side where the AC / vent draws air through the kick panel hole, I don't think there is a screen. I recall several years ago removing the accordion like rubber to clean the cowl vent bowl that has a hole pointed towards the ground. I know I removed the sliding door, etc on the driver side too. My truck still has an evaporator blocking a critter from the rest of the AC ducting, but a critter might get into the cab that way. I don't see much getting through the cowl vent slots in front of the windshield, but a mouse might somehow get up off the ground and up to the cowl vent "well" that has a fairly nice size drain hole at the rear corner, near the lower door hinge.

I can't think of any other places a mouse might get in, but am open to suggestions.

Update:
Well, went out to try to sniff it out, can't smell it now. I know odor was there, Wife smelled it too earlier.

 

I had a mouse nest in that driver side kick panel vent. Most likey, IMO, that might be one area where it came in. I found mouse evidence in the interior roof channels where the headliner trim screws in. My guess is that they squeeze up the A pillars.

 

I contend with field mice too. My defense system is peppermint spray and bait blocks (Tom Cat brand). Seems to be working for me as I haven't had a mouse in any of my vehicles for awhile now. I cut up the bait blocks and place them strategically around the engine compartment where they will easily find it. Last resort....you might have to trap it to get rid of it. I hate rodents...