Well, for starters, make sure you have the knobs for the heat controls. If you have digital climate controls, you cant add an aftermarket head unit with an adapter.
(some would argue with me on this point, it is possible, but not worth the aggravation or the $$)
If you do have the knobs, you need to get an adapter for the heat controls. Check the classifieds on this site, or search ebay or other places on the web.
A full kit with wiring harness will run you about 70 bucks.
-DC