You do need a little heat to get film off in one piece and prevent it breaking / tearing. However too much heat no good either. With the way the current weather is, leaving it in the sun for an hour should do and saves you getting the interior wet with a kettle etc. Just need to have a blade to get a corner started and will come off easy enough - but... will leave glue behind no matter what you try.
BTW, to minimise strain on surface underneath (like your demist tracks) pull the film back on itself, like a really sharp fold like. That way the strain is at 90 degrees angle from lifting off the glass and pulling along the glass. It's the best you can do to try minimise damage.
After that it is raggs and solvent of your choice and a lot of elbow-grease