The heated rear windshield is not working. Also, when I take the car through the car wash, the passenger's side floorboard gets flooded.
It appears that you may actually be experiencing two different issues with your Mustang. I do not believe the two are related so we will look at each concern independently. In regards to the rear window defroster, it is quite possible that you may have just blown a fuse.
If it can be verified that the fuse is fine, then the next place we will want to look will be the lines on the rear windshield glass. Those lines are actually heat wires hooked up to the electrical system. When power runs through them, they heat up, remove the fog from the window. Over the years these wires can lose proper connection. This is often through the abuse of the sun beating down on them or possibly an object made contact and created a break in the connection. Due to this failure being an electrical issue, I highly recommend enlisting the help of a certified mechanic to diagnose and repair this window issue to avoid any other electric failure.
In regards to the passenger side flooding while going through a car wash, I have actually found this to actually be quite common. It is found in the newer Mustangs and the fix for this is simple. There are grommet seals that go under the windshield and these seals hold the cowl (a plastic piece found under the front windshield which covers the wiper motor system as well as housing the fresh air for inside the cabin of the vehicle) in place. What you are experiencing is when these seals go bad. Ford sells an updated seal to correct this issue and the new seals require heat to be applied for a proper seal. While the repair itself is not too difficult, it does involve removing the cowl to gain access. A certified mechanic can also assist you with this procedure and get your Mustang’s defroster and seal concerned taken care of quickly.
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