My A/C smells musty!

First, if you have an in-cabin microfilter, ensure that it is clean and functional and not backed-up by debris, which can retain water, causing the musty smell.

Next, with the Xterra running, and the A/C on with moderate fan (disable "re-circ" mode) - spray Lysol Disenfectant into the grille/openings on the windshield cowl, just below the bottom of the windshield. Apply a liberal amount, then shut down the truck and A/C and allow to sit for a while. Repeat if necessary.