1999 Mystique Trunk Frozen Shut

I have a 1999 Mystique with its trunk frozen shut.  It's not broken, because I was able to open it this weekend when it was 45 degrees out, and this has happened before when it gets cold, so I usually just let it warm up outside and it unfreezes.

The issue I have NOW is that my tire is flat, and the car is sitting in a gas station parking lot.  I spent 45 minutes this morning trying to open it, I poured anti-freeze and hot water (understanding that hot water could re-freeze it, I just need to open it once to get my spare tire and jack, so I'm not worried about it freezing afterwards) into all the cracks around the side, top and bottom of the trunk, but to no avail, it still wouldn't open.  I ran the heater inside for about 10 minutes, but I had to get to work, so I got a ride and am going back later today to try it again.  

I parked it in sunlight, but it's not going to get above freezing today; is there anything else I can try?  The latch to fold down the rear seats is in the trunk, but if I could even get in through the back seats that would be fine too.  Is there somewhere else I should be pouring anti-freeze to try and melt whatever seal is frozen shut?  Would running the heater on full blast for closer to 30-40 minutes make a big difference?

Thanks!