STEAM??? I hope not!
I hope you are referring to condensation and not STEAM.
If it's true blue STEAM, you got a SERIOUS problem, not only do you have a hole in the heat exchanger core, but you also are seriously overheating!
If it's condensation, then the problem may be that the leak has permeated the matting under the heater box, carpet, or vynil on the tunnel, and is now evaporating and condensing on the first cool surface that it finds, your windshield.
Since I'm guessing that it is just humidity inside the car that is condensing on your windshield, here's what I would do.
First determine WHERE it's leaking, this could still be either the heater core or the valve or the hoses. SOMETHING is leaking and providing the water for evaporation to occur.
Once you've found the leak, fix the item accordingly. That is, Heater Control Valve: don't waste your time and get frustrated trying to fix the old, get a new one and replace it.
Hoses: Ditto, it's cheaper in the long run to replace worn or cracked hoses with new ones than to be continually patching.
Heater Core: Remove from the car, take to a radiator shop and see if they can fix it, but check the price of a replacement one whether from a reputable boneyard (i.e. some sort of warranty) or a new one from Motorsports, Victoria British, or Nissan if they have it. It might be that it's cheaper to replace than to fix.
Next, you need to dry out the carpet and anything else that got wet. Since you probably have anti-freeze in your radiator coolant, I would recommend you remove the item from the car and rinse it first. Yes, get it further wet, but with CLEAN water. That's because today's anti-freeze liquids aren't just ethylene glycol, they also include lubricants and preservatives and other stuff that will NOT evaporate. If you leave the carpet pad or vinyl to air dry, it WILL take WEEKS, and never really dry out.
By rinsing, you'll get the bulk of those other items out. Then carefully squeeze out as much water as possible, and either hang or lay flat to dry.
Recently I had to dry out the passenger side carpet on my car. Since I'm getting it replaced eventually, I wasn't worried about how to dry it. I came upon this method and it worked very well, after soaking the carpet and backing, I carefully rolled the carpet up like if it were a sleeping bag, squeezing out all the water as I went up. Once I had all the water squeezed out I could, I then clamped them to a sawhorse I had, so that they hung down. Within a few minutes the last of the water was dripping off the carpet. As it dried, it straightened the carpet back into shape. You can also use a room fan to blow and quicken the process. I wouldn't use a heat source unless you're real careful.
Well, I hope this helped.