Even if the ductwork isn't easily accessible for replacement, most air ducts are large enough to be cleaned with appropriate equipment, although it's not usually a DIY job. If the duct stinks, it means there's contamination in it; and the same kinds of contaminants that produce odors are also prone to producing "infections" that can be a very serious health hazard.

BOBERT HAS IT RIGHT: the ductwork needs to be professionally cleaned and/or replaced.

The cleaning should give some indication of the source of the odor, which may be anything from a dead animal to accumulated dust or to a "condensate" problem with mold/mildew. Depending on what's found by the cleaning, some additional corrective action may be indicated to prevent recurrance of the problem; but nobody can predict what will be found until something is sucked out and identified.

OF COURSE the initial assumption that it was "pee on the back steps" is a cause for our concern about Bobert's level of cultural/social acclimation and behavioural deviances (who did he think might have pee'd there?) but his (so far as we know) accurate assessment of the real problem, and immediate selection of the appropriate remedies speaks of intelligence and clear insight on a par with the best of us here, despite being somewhat unexpected in an illigit unsophisticated hillbilly of the kind he affects to be.

And we should not hold it against him for not having total control of his wet spot(s), any more than we blame Bill D for having no functional nipple. My money's on Bobert to solve that wet spot problem (if he thinks it worthwhile) long before Bill D makes his nipple work.

John