2014 mustang convertible water leaks

There is a gutter in the trunk below the bottom edge of the top. Make sure the gutter is not damaged and that the drain hoses are still hooked up and drain outside the trunk.

Just a suggestion but maybe you need to start hand washing your car? Convertible tops and high pressure water are not a good combination.

The 2014 mustang does not have a gutter. It uses a floating rear sealing bar, and several rubber seals attached to it. These seals can be damaged or have debris stuck to them making a poor seal. The top material and seals can also detach from the bar. If the top material is taut, and not sagging around the rear window. It is still attached to the bar. It is stapled to the bar and can come loose. You mentioned water getting in behind the rear side first then getting to the floor after sloshing around. Check the rubber weatherstripping at the bottom of the window on the side you hear the water sloshing on. They can become damaged or come off all the way letting water in. Lastly Check for damage to the soft top material directly behind the rear side windows. You can do this by slightly lowering the top and look at the top material where it butts up against the quarter panel seal. There is trim that runs around the body opening at the rear of the top. The small pieces on the quarter panel are held on by a few clips and three studs welded to the body. I have seen on three occasions where these metal studs have poked through the top material. leaving holes right near where the rubber seal contacts the car. Regular rain can get through them but not always. The high volume of water in the carwash will get in. It is also a good idea to periodically treat the seals around the back of the top with silicone spray. The seals will get hard, or stick to the bodywork if you don't open the top regularly. Again you can do this by lowering the top between 1/4 and halfway. Whatever allows you the best access to the top seals.

I sent along this photo hoping it would help explain what areas I am talking about. The area that looks dirty is the part that's tucked under the body lines. This picture shows the seal area too

Water collects in the side panels in the rear quarter. If you have the top down you will see the water if u look with a light inside the quarter panels, it has drains in the pinch welds between the rear wheel and the door opening, use shop air to clear the drains, or a drill to drill new holes in the pinch area