Sister-in-law's cat was staying with my inlaws... My MIL took her Marathon out of her car for a couple of weeks... Cat peed IN it...
What should i use to wash/remove the pee/smell?
The same enzyme based product that you'd use on carpet or upholstery on the *cover* only (not the straps!!! the enzymes can break them down and lessen their safety!!! new straps can be purchases from britax if needed.)
Nature's Miracle (white bottle with red lettering) or Unique are my two favorites for pet urine. NM has a special one just for cats that works better than regular NM. Unique works better than regular NM, but the same as the NM one for cats. Follow the instructions to let it sit and soak, full strength, with the enzyme solution on the stained areas. It may need to soak for an hour or two...and may even need to be repeated...but I doubt it. And then wash and rinse as usual, add an extra rinse. (I'm a dog trainer and animal rescuer, so I've gotten urine out of a lot of upholstery thru the years! LOL
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The enzymes will break down the phenols in the urine to get rid of the smell.