There are two slots under the bottom edge. Get a couple of narrow handled teaspoons and push them in. It might need a bit of fiddling about and jiggling, but it worked for me.
If you have a Phase I/II, even if you break the plastic, it can't be removed easily; if you can get hold of the 306 Haynes book, there are templates so you can make the tools yourself.
If you have a Phase III, it's the two holes next to the CD slot. Push, hook and pull (with a generic ISO tool).
Just about every 2nd-hand Phase I/II 306 I've seen with an aftermarket stereo has its radio slot damaged through using the wrong tools for the job and by consequence: the evident frustration of not being able to remove the radio easily. So take your time and use the right tools to avoid burring the plastic.
