Maybe the problem is that youre not pushing them in far enough. When you push them (nails, coat hanger wire, whatever) in the holes youre actually flattening four little spring-up clips.
So you need to push it in at least 3/4"! Then youll feel some resistance as the nails/wire hits the clips. You have to keep pushing and it will slide past the clips. Then therell be no more resistance. Once youve passed the clips, the HU should be free to pull out of the dash.
You might need to hold the nails or coat hanger wire outwardstowards the edges of the dash while you pull outwards so that the CD player comes out too rather than the wire/nails just sliding back out of the holes.
Alternatively, push in the wire/nails and leave them there. Then, try to grip the HU by the CD slot and pull it that way.
It should take about 5 seconds!
Scott