Removing the Headunit
As previous people have said you need to be forceful. I have removed mine twice now and both times had really big problems getting it out. Both times I have been removing it with a professional car stereo installer. The installer has removed heaps of them and said that mine is the hardest one yet to remove. Some seem easier than other to remove, the first time it took us over 30 minutes and the second time it took around 20 minutes.
You just have to keep at it, all you are doing really is pushing in 4 pins to release the unit, best thing to do is use bent coat hangers as all the other people have said, but use 2 people, put the 4 hangers in wait until they all click then all at the same time pull the hangers to the side of the unit (left for the ones on the left, right for the ones on the right) and jiggle them about.
Both times I did it this way and we thought we were getting no where and just when we were about to give up the unit popped out. Having 2 people seems to help alot.
Hope this helps.