Blast it with WD-40. It might take a few shots over a couple days. Heat MIGHT work but you've got to be careful not to burn the plastic handle and surround. Failing that, have a body shop pop it out and take it to a locksmith.
I had to do that with both the door and trunk locks on a used Mustang I got for my son. Apparently the previous owner just used the key fob all the time, never used the key, and they corroded and locked up.