Dead critter somewhere in the heating system 95 Pathfinder.

There's no way a critter larger than a small bug can get into the ventilation system. More than likely, it is dirty water trapped inside the heater air plenum. Water that gets into the plenum is supposed to drain out through the firewall into the engine compartment and then down to the ground. It may have debris clogging it preventing the water from draining and causing the water and particulate formenting into an odious tea inside the plenum. You might try cleaning the plenum drain. It's a short black molded hose with a 90º downward bend sticking out of the firewall on the left side inside the engine compartment. First, just try squeezing and manipulating it to loosen the debris. If successful, the water will pour out. If not, you may have to pull it out of the plenum from inside the car. Have a pan or something and a big towel to catch the trapped water so it doesn't spill on your rugs.

Might also try using a coat hanger - putting it up inside the hose and trying to dislodge any sludge. I had to do this with a Toyota pickup I had once.

Sure there is. I've found dead rats inside blower motor housingson several cars.. No idea how they got in there but were there none the less.

You do have a point about water, but it's in the A/C evap not the heater box (plenum?). Still can smell bad!

To the OP if the smell in only on fresh, it's something to do with the cowl area of the car as that's where the fresh air is pulled in, you should be able to smell it outside the car as well.