I've done about 10 sets of different lights now, it's all very well heating them up, but then they are far too hot to handle. Your best bet is to use a heat gun to warm the glue section at a time, nice and hot, just before the plastic starts to melt. then run a blade or a small flat blade screwdriver through the glue. You should then slowly be able to pull them apart.

Does make a huge difference, here's my current car, before and after.