Need helping installing single din in 05 CTS
 

so as the title says, i need help installing my single din radio to my 2005 CTS.
i already have aftermarket speakers in all 4 doors. i bought a wiring harness as well as the dash kits from sonicelectronix.com , and they said they are made to fit my car. lo and behold they dont. apparently the harness i received is wrong although crutchfield also says its the right one. (metra 70- 2054).
we already cut the stereo out with the dremmel so if anybody knows which harness im supposed to use or if someone can give a whole step by step explanation? ive watched videos but mainly everybody uses a double din so its slightly different.
thanks in advance, im losing my mind here.