Have never done it with Lexan, but generally you would heat both components together until one assumes the shape of the other (given that the mould must be able to stand a higher temperature than the moulding).

Attacking the Lexan with a heat gun is unlikely to work because of the heat distortion point being well over 140 deg C, you would never achieve this uniformally on the piece you are considering, especially as your mould (glass panel) is a very effective heat sink.

The best way I can think off is to find someone with a large enough oven (definately NOT the one in your kitchen ) that will go to at least 200 deg C and heat the Lexan & your sun roof panel together in an oven till you hit the "sagging" point of the Lexan (probably approaching 180 deg C), when it should "sag" and assume the shape of the mould, then allow to cool before seperating. However you may need to provide "force" in the way of a vacuum bag to cause the Lexan to deform, which makes it a bit tricky.

Sounds more and more like a job for the proffessionals but do read the guide here... http://www.bayplastics.co.uk/product%20guide/Altuglas-guidenavbar/prod-altuglass-tech(thermo1).htm

Cheers, Sim