doood - it might be backstrapped. take off the center console under the radio and you'll be able to see underneath the radio. the OEM radio has 2 8mm bolts that hold it in, it's possible that whoever installed the Clarion HU put some bolts in the bottom of the radio chassis, or they put a backstrap on the radio and screwed it into the firewall or something. just get underneath there and take a look. you'll be able to see what's going on once you get under there.