L.P.
If the cover removes, and I don't know any car seat that it doesn't, just take it off, wash it with tea tree oil and vinegar. One is a disinfectant and the other removes musty and moldy odors.
You could also do it with a hose and laundry soap, but it would take some scrubbing and lots of rinsing if you leave it together to do, but you don't have to put it back together that way.