I bought a GromAudio kit for my stock head unit in my 2006 xA. I want to pull the head unit out so I can attach the GromBox inside. Where do I begin to take out the head stereo unit?

 

First you need to remove the silver side trim, read this post to learn how to do that.
https://www.scionlife.com/forums/viewtopic.php?t=37733

Unscrew the screws that are shown in the very last picture of the above post. Then grab and pull out the three A/C dials (just grab them and yank, they'll pop right out). There should more screws hidden behind the dials, unscrew them too. Now grab the whole A/C part of your dash and pull out. AC control area, air vents, and HU area are all one piece that is held in by plastic snaps, so just be gentle with it and it will pop out. Here is a pic of what it looked like when I popped mine out, you can see its all one piece (In this pic I have already taken out the HU). Once you have done this just unscrew the 4 screws holding in the HU. *Note: remember to plug everything back into the HU once your done, there is nothing more "unfun" then putting everything back together and then realizing that you forgot to plug in your steering wheel controls. And yes, I speak from experience.