Last year in late fall I put a coat of meguiars ultimate liquid wax (a polymer sealant) on and did not buff it off. You could barely see it.

Water beaded all winter long without washing. Dirt just rinsed off.

Do you have cloth or vinyl top? You have to get the right product. I use the blue magic vinyl top cleaner & conditioner.

 

My 2 cents...in Cali its rarely too cold to wash your car, however, I will occasionally run my car through the car wash and never had a problem, until last week when about a half gallon of water dumped onto the floorboard of the passenger side floor mats.
I immediately came home and googled the problem, and thats how I found this sight. I have since solved the problem. There are two drain holes under the plastic cowl between your windshield and hood. Removal is easy (other than the drivers side wiper arm removal) but they can be cleaned out from the engine bay. They are rubber, tea bag looking filters, for lack of a better description, near the firewall on each side that are accessible to reach up inside from the bottom and empty all the twigs, leaves & pine needles from. No more water leaks.
As for the car wash, I tell them DO NOT use the wheel brush on my paint! I frequent the same car wash, tip good, and they will do whatever extra detail I ask for, never had a problem with rear window leaking. The water & blower on the top is a bit loud and annoying but it is also a good way to make sure your seals are working properly (as your part of country has much worse weather we do)
And a word on convertible top protection...the one time I used a product on my top, it left chemical streaks down my door windows every time it rained or I washed my car. Streaks I couldn't wash or wipe off. Over time they came off, but it took a long time to get them off. I would love to find a product that doesn't do that?
Just a chicks opinion from her experience...

 

I run mine through the auto wash all the time, no issues. The only time I had a problem was after I had some crash damage repaired and the windows weren't lined up properly. Took a couple of hours in the driveway to line them back up (sacrificing a 10mm wrench to the crevice gods in the process) and the problem was solved.

I'll have to check the cowl filters, I get a lot of pine needles falling from the trees.

 

By chance do you have a sunroof?

 

Check your cowl drains, make sure they aren't plugged up. I had a similar problem with a Cavalier, the cowl would fill up and throw water on the windshield when I turned the defroster on.

 

I just got a 2001 Mustang GT convertible the week before Memorial Day weekend, my thought on the carwash thing is do it yourself if you can. I know like myself we don't all live in an area that's warm all year long, I also have another car to drive in bad weather so mine shouldn't get too dirty in the colder months. I just wanted to know what is good to use to clean the top? I have a black top but it still gets dirty even if it can't be seen easily.

 

I vacuum my cloth top with a medium soft brush and a light pressure. If water is needed, I just use a hose and spray nozzle then the wet vac.....

 

I have used an automatic car wash several times with no leaks or other problems. I wash it by hand most times, but when it's really dirty, I use the car wash. I also get my oil changed at the car wash with Mobil 1.