Do not use trial-and-error when attempting to open the dash on most modern cars. You can damage the plastic if you don't know what you're doing.

Call Crutchfield and ask for a MASTER SHEET. For some cars, these are worth their weight in gold. If I had not gotten the master sheet for my Accord, I would have seriously damaged the door panel when working on my speakers, and probably damaged a dash panel when installing the stereo.