Austin, sorry I was right
Over the years, we have tried herbal and liquid deterrent, poison, mothballs (and castrating all those moths was tough :lol: ), taking the heaters inside for the winter, etc., ad nausium :!: The bottom line is that mice will go wherever they can despite deterrents if they NEED a place to live. Last year I finally hit on something that
will stop them - steel wool. (I've heard 2 theories on why steel wool would work: 1) mice don't like the taste of blood and, in trying to chew through it, they cut their mouths 2) They eat the steel wool and cut their innards to pieces and die

)
Pack the holes that the wires feed through and any other cutout larger than 1/4" with steel wool (these little b*astards can get through a 1/4" opening) and tape up the blower air feed for the winter (you have to leave it open for the unit to operate, so untape it in the spring) - I did this to ~ 20 pools last winter and all of those we've opened so far have been mouse free

Don't worry about the casing itself, too many places to try to block, just worry about where the electronics are.
Now for a $ saving tip from Ted - You can wrap electrical tape where they chewed, as long as they didn't chew through the wire completely, and the heater should work again, no new harness required :-D