Does the fog dissipate rather quickly or does it still hang around? That's key. If you've just cranked up the car and it's not warmed up yet, you are just blowing cold, moist air around and it will condense further on the windshield if it's really cold out. If it is already hot when you do that, and doesn't dissipate, that's not a great sign. And does it do it every time?
Do you still have the foam seal around the heater core piping? Check to see if it's wet. I had a beater 72 Cutlass supreme one time [yes, it was dark brown. I think 98% of the 72 CS's non-H/Os were] that had a leaky hose connection that seeped water into the heater core area along the pipe (apparently the firewall seal wasn't all that and a bag of chips) but didn't have a visible external leak. I went through hel l changing out a heater core that didn't need it. I'd rather change out a heater core on a G-body than an early 70s A-body.
Check the bottom drain flap on the HVAC housing to make sure the drain is clear and see if there's anything coming out of that flap that resembles coolant. I'm not saying it's a problem, but it sounds like a start to a leak that will get worse over time. Sometimes they just start out as a seeping leak that only gets worse over time. Flip the heat over to the upper vents directed toward you instead of defrost next time you crank up the car and see if you can smell any coolant at first air burst. That's when it likely will be the most concentrated.
And cross your fingers you won't be popping the top of your HVAC housing having to change a heater core any time soon. Good luck.