Oh, my goodness. There were about to be multiple melt-downs over a car wash !!
I can see where a true touch-free car wash may be okay but I can just see the antenna or something getting ripped off a car at the Goo Goo.
When I lived at an apartment complex, I had a connector on my kitchen sink and a 100-foot hose to wash my car in the parking lot.
I'm guilty of taking a wash bucket, soap and mitt with me on vacation ( or buying if I forgot it) to get bugs and road grime off my car when I get where I'm going.
That said, these cars were prone to leaks when new, let alone after 50 something years. I will occasionally spray too hard at the sides of mine and get a little bit to come in around the side or vent windows. Oops. I'm not sure they can be made to be 100% leak-free. That's part of why they rust so easily.
I would recommend as others have to become or find a contortionist to lay under the dash and look for the source of the leak with bright lights or water, and make sure the windshield is sealed with a tube and a half of 3M B&G C. Otherwise, the leak may be coming from a place where seam sealer has lifted off. I fixed that in an otherwise perfectly solid cowl by using a 360° spray wand to spray sealer on every surface and pouring some in through the taped-up cowl air intake to the point it spread over the entire cowl floor and actually ran out the side drains behind the fenders. No more leaks. Good luck!!

 

When I lived at an apartment complex, I had a connector on my kitchen sink and a 100-foot hose to wash my car in the parking lot.

Man, that's awesome.

 

You take your car to a carwash?

 

Sorry, my bad. No carwash obviously. I just meant washing my car on my driveway.

 

Pre '69 windshilds should be installed with the Bedding & Glazing Compound, a 3M product. Your better stocked Mustang parts houses usually carry it. Occasionally you might find it at chains like O'Reilly's. Even if the the glass and rubber gasket is "installed" correctly, keep in mind that those spring loaded clips that hold on the stainelss trim around the windshield can be a source for water leaks. Apply a generous dollop of the bedding compound around each clip before installing the stainless trim. That will usually do the trick for those difficult to find leaks.

 

If it's dripping in the middle of the footwell could it be leaking in at the windshield wiper seals?

On the passenger side kick panel check that it's not coming in on the antenna wire or leaking behind the hinges.

 

When I had a shop install my new windshield seal in my 67 they had it back 4 times and couldn’t get it to seal. This was a mustang restoration shop. That’s all they worked on. The last time after it didn’t seal they just gave me my money back. I’ll do it myself on the 66.

 

They are really hard to seal. I had a reputable glass shop who does classics straight up tell me they would do my windshield but would not guarantee against leaks. My local mustang shop wont even do it. They send them out to someone else because they don't want the hassle.

To the PO - I'd wager you have either a leaking windshield gasket or your drop rails are leaking. If its coming down the kick panels its probably drip rails.

Also did you say your take your mustang through a car wash...?

 

So my 68’s cowls are solid (even coated in there by someone) I never had water leaks in my front windows until!! I drove through the remnants of a hurricane coming back to VA from NY Window gasket leaked at the corner at speed under heavy rain…. Stuffed tshirt in it eventually but at first it started pouring at my feet. Cowl had weather covers over it. So first thing I’d suspect is the window gasket. Get a hose on different parts of it and try to find the area of the leak.

if you do replace the windshield gasket… And don’t do it yourself make sure to go to a place that knows Mustangs glass specifically. If you go to a shop that uses modern urethane sealants it’ll leak like a siv - went through this on my rear window seal. Had to bend it redone. 1st guy was using totally the wrong stuff.

 

Are you sure its not the windshield? Even though it was re-installed.
May want to remove the kick panels to get a better look. Get in the car with a bright light and have someone else spray it with a hose (from the outside).

 

Yes that's what I tried and why I suspected it to be the windscreen, but it was professionally re-installed. I know, they can do bad repairs too, but from the images they shared, it did look like they did it the right way.