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The amount of buildup depends on the amount of rabbits she kept in there. You mentioned plural, and even though two rabbits often don't share a cage together very well, it is possible she had more than one in the same cage. Rabbits will usually only go in the one corner of a cage, usually the furthest away from the food. But when rabbits urinate, there's a lot of it! And Candice is right, they do drink a lot more than many other animals, mostly because their food is mainly dry pellets.
A friend of mine, who used to have ferrets in one room (they were allowed to roam the house, but were fed, and had to sleep and do their bathroom business in the one room), sliced up a potato and put it on a plate in each corner of the room for a few days whenever she had to clean out the room. It did the trick of removing the nasty smell of urine and the potato, by the time it was done, looked like someone had peed on it!
I'm not sure if this trick will work with plastic, but if you've got a potato kickin' around, it might be worth a shot.
Good luck getting the smell out!