That is a really good question.  There is a button on the light switch you need to push that in and then pull the light shaft out.  It may have to mess with it a little but it will come out.  When I tried doing it I almost gave up because it just did not seem to want to come free.  It will just hold the button down. and jig-gel it as you pull it out.

By the way before you put your new dash back in decide if you are going to keep the radio. The speakers are different for the new radios and you do not want to pull your dash to replace them.  Also you need to check them and make sure they are good for the same reason.

Those things can be difficult to get to release..I think I did the last one by feeling when the lock on the bar goes down just a little bit then gently wiggle a little..by the way it has to be all the way out to get it to release..
Rich

The release button can be hard to see/find sometimes, but it's there! As Gilbert and Rich said, you may need to jiggle the shaft/knob a little while pushing all the way IN on the release button. The light switch should be all the way ON first, so be sure to disconnect the battery, if you haven't already.

The pin can be difficult to find at first and then difficult to push in all the way.  Just keep trying and be patient which is hard to do when you are crammed under the dash of the car.  The pic that Joel posted shows the location of the button.  Im posting another pic, although it is poor quality that gives another view of it and also a pic of the headlight shaft so you can see the shoulder at the end of the shaft that the pin engages to hold it in.