Guys, how on Earth did a mouse get into the Heat /Air box area? (1995 model)

Mice have always tried to take up residence in and on our Blazer. They've never been inside but there has always been lots of evidence of those critters on the engine as well as chewing away the sound deadening on the bottom side of the hood.

Well, about a month ago we smelled strong mouse pee when we turned on the AC. It was horrible. Running the heat never helped, made it worse. So, running the AC pulling in highly humid air finally flushed out the odor. It was fine until three days ago when the odor come back. Then two days ago the wife said she heard something like debris hitting the fan. Well, this morning it obvious the debris was a mouse. That darn thing is in there dead.

So, as with all these cheaply made heater boxes, the outer shell has all but fell off. The inner shell seems solid. So, where did that mouse get in? Is it possible it accessed the heater/AC box through the drain hole?

Thoughts?