I'm guessing that Pioneer installed down below might have been the original in-dash unit, since I can't believe the Alpine came from the factory. Not even sure they installed XM antennas at the factory....does the Pioneer have XM capability?
I had a guy install MY XM antenna on the roof directly over the frt. left side ceiling speaker. That way its easy to route the cable into the overhead cabinet over the driver's left shoulder, then into the nearby pillar and down to the dash area. Maybe you can remove the speaker and begin tracing the cable over to that compartment...etc. If it's an after-market install, the cable/jacket may look a bit different to factory work and would be more easily identifiable.
I like Dick's suggestion as its similar to what I did with a GPS antenna on a Pioneer AVIC I recently installed. I just tossed it onto the dash in front near the bottom of the windshield and it works great. Don't see why the XM wouldn't work in the same manner. A little double-sided tape would hold it in place.