this is a 91 spider clarion........you can see the 2 holes on the far right for the tool (same on far left, but not seen in photo)
is your clarion different to this photo?
If it is different, you probably need the flat keys.
I hope you don't mind me jumping in to this thread 6 years later. I want to remove my S4 OEM radio as pictured in your photo in order to repair one of the A/C vents. I've tried wire hangars, finish nails and although I hear clicking the radio won't budge. I even removed the carpeted panel on the passenger side foot well but couldn't get enough leverage to push it out. Any suggestions?