I have a 2005 XJ8, 48,000 miles, that runs fine. The antifreeze level stays constant, except for now that it is winter again. I have found that, if I use the interior heat, the windshield will fog up on the inside after sitting. When I don't use the heat it does not. Antifreeze level drops after heat use yet does not if no heat is activated.
I figure that I have a heater core issue. I do not know if it is a hose or core issue. I wonder why it only leaks when the heat is activated.
Taking the dash apart is a huge issue but I think that is where I am headed if I want to have heat.

 

Quick check your carpet passenger side .if wet heatercore or hoses leaking.

 

Thank you, checking the floor mats for dampness was the first thing I did when I first saw the condensation on the windshield interior. I did this repeatedly whenever I saw the condensate. It was always dry, telling me it is a very small leak. At first I thought, and still haven't ruled out, that the windshield seal has gone bad. It maybe a leak in the sunroof that somehow channels it's way down the sides of the windshield to the dash area.
I almost never use the heat but had been the past few weeks. My antifreeze was about 3-6 ounces low. I stopped using the heat and haven't had a fog issue yet. The antifreeze appears to be holding level now but I almost never check it. I don't know if it was a sudden drop or a very long slow leak.
I am going to keep an eye on it for now. The heat will stay off and I will check the antifreeze level regularly. If the problem remains at bay, even after heavy rains, I will try the heat again and see if it comes back. If it does, then I have two options. One; bypass the heater core. Two; Take apart the dash and get to the heater core to repair the hose or replace the core.