Best way I found was to jam a big flat blade screwdriver down the left hand side of the headunit and just leaver that side out then use the blade to push the 2 pins in on the other side, found out after a big strong knife works just as well, got mine out with no prob's
only problem comes when fitting new one as if you don't slice the origional wires and use a adaptor harness like iv'e done there is nowhere for the wires to go, and you end up with the head unit standing 2" proud of the centre console, what you have to do is leaver the air vent up of the floor and tuck the 2 big connector blocks under there then you can carefully tuck the unit in place flush.
hope thats of some help to you
P.S. do not even think of putting a amp onthere unless your RCA's on the head unit are on a short lead from the back as they will not allow the unit to sit flush, I use a Alpine head nit on which the RCA's are the Sub out which are on a little lead, just tuck them under with the connector block.