Well, did you try opening the sunroof and pushing on it lightly - medium with your hand?
Basically:
- Press the one switch to open the sunroof
- Lightly press up and back if you want it to open all the way, or just press up if you want it to just go up a little.
When I first bought my 99 RS, I had the same problem, sometimes the sunroof just gets stuck in the seal.
As for the heater core... I'm not too sure on what to do with that, as I THINK you have to at least take out the airbag / lower dash on the passenger side car to even get to it.