The way the power harness for the GEX-P920 is setup you have the yellow power wire at the same end that plugs into the radio, and the ground at the opposite end where it plugs into the module (in order to keep the ground lead short). You want to tap the yellow into the yellow constant wire off of the radio which goes to the orange constant wire in the vehicle harness. Ignition would work but you would lose your XM presets everytime you turn the vehicle off so definately tap it into constant. The reason they mention not to go to the battery is for a couple reasons. Mostly because it isn't necessary and increases the risk of noise. That tuner draws very little current so tap it right in with the radio and you will be fine. On the other end like you I always mount the sirius/xm tuner right in place of the OEM tuner on the passenger underdash panel and use the OEM antenna. I pull the kick back slightly and ground in the kick. Being such a short run you could get away with extending the ground to tap in behind the radio, or figure out which wire is ground in the OEM tuner harness and use that. And another thing that throws people off when hooking them up. The end with the blue ipbus goes into the back of the radio, and the black end at the tuner plugs into the black plug. Mix those up and everything recognizes but no audio from the Xm.
