There was a rusted out hole on the side of the cooler that allowed the rat to enter. You can see the hole on the lower right corner of the picture below.
The owners of the home had a dog and the rat had carried in a fair amount of dog poop into the nest. Some is visible if you look closely. Collecting dog droppings is an interesting behavior common with pack rats that seems to make no sense. My personal theory is the rats do it to discourage bobcats.
Fortunately the owners had a damper in place that blocked the rats from entering the duct work of the house. I have seen other nests on duct work and just imagine breathing the dust and other matter from a nest like this one. You really don't want pack rats in your duct system.
This cooler was totally shot from the rust and rats and had to be hauled off after trapping the resident rat.