I went through a car wash this week and remembered how my son used to be very afraid of them. I thought this video might be helpful for others in a similar situation. You could watch this video to help prepare your child for going through the car wash. A great option about a video like this is that you can control the volume. If the sounds might bother your child, you can watch it with a lower volume the first time, then gradually increase the volume if you watch it multiple times. I'd also narrate what is going on during the video so that the child knows what to expect.
Matchbox Car Wash Adventure Set
Car Wash Bath Toy