I don't trust factory grounds, so here's how I did it.
I gathered the AVIC ground wire, parking brake wire, bluetooth ground, camera ground, and Sirius ground and soldered the ends together, along with ~18" of 10 gauge wire to extend the ground lead. I hate extending grounds, but I swear my interior is entirely made of plastic. I soldered a large washer to the other end of the 10 awg wire (homemade ring connector). I found a hole in the chassis near the gearshift, so I sanded the paint off, screwed the washer down on the chassis with a large metal screw, and called it a day. It holds nice and tight.
Since the cell phone mute wire is pretty short, I cut the connector off and used one of the Pioneer-provided wire taps to connect the mute wire to the parking brake wire. Since this wasn't a part of the main power supply circuit, I figured there should be no problem tapping one of the other grounded wires to make my life easier. It worked like a charm, everything functions in good order, and the only thing I have to worry about possibly falling off is the mute wire tap. Not the end of the world, shouldn't short anything out, just disable the bypass if that happens.
For what it's worth, I don't trust the factory power either. The fuses of all my components add up to 15 A, the stock radio is on a 7.5 A fuse (the AVIC alone is 10 A). I ran a new 10 awg (fused at 20 A) power wire from the battery to the radio cavity, where I connected the various power leads. The AVIC reports a steady ~14.3 V, so I figure my connections must be fine.