I now own a '05 EX 2WD
I have a couple of questions that I was wonder if anyone knew:

1)Is it possible to use the built in XM equipment in to run XM on an aftermarket HU? Can I use the stock XM antenna? Is there some sort of adapter?

2)Were in the hell is the antenna port on the back of the stock headunit? Is it integrated into the XM antenna port?

3)Is the big bar of metal running behind the HU that looks like a reinforcement bar able to be used for a ground for the HU?

4)What the hell are all the plugs on the back? I know one is for speakers/power, but what about the rest? There is a black one with 3 wires, a green one, another that I believe is the antenna, and a white one.

I have a Pioneer Premier 860MP and bought the Terk tuner and adapter for XM. Can someone please help me

Question 1 - I'm not sure. I have an '04 so I dont have the stock XM headunit. I have heard others talk about an adapter that you can buy for XM use with an aftermarket stereo, but I dont know much about it. I believe someone was asking that question about a week or so ago and mentioned that Metra was making an adapter. Not sure if its done yet or not though.

Question 2 - Possibly. The stock FM/AM antenna and XM antenna are one in the same on an '05, but I'm not sure about the antenna plug on the HU. I know the guts to both antenna are actually in the antenna, but I'm not sure how the wiring is run into the headunit.

Question 3 - It is possible to ground out there, but if you can, use the ground that the stock stereo uses. Grounding out on that piece of metal has the possibility of being a pain since its not that large, you'd have to sand a bit on it to get a good connection, and you'll have a huge amount of wires rubbing around right net to your ground - which can seriously fry some stuff - but that's worst case scenario.

Question 4 - You have one more plug than I, so I assume that has something to do with XM. One plug is for your sub's signal and the other is for your AUX. Look at this wiring diagram. Each harness is broken up and shows what wires are leading to what:

http://home.comcast.net/%7Ederek-kelly/element/audio/eaudint.jpg

All in all, I'm not much help on the XM area. Hopefully some of your other q's are answered though.