Welcome to the Alpine Forum! You just bought what we all think is the best handling coach out there!
Start with the panel directly in front of the passenger. It's about 2.5 to 3 feet tall and about that wide. It is below the "dashboard". As I recall, ours has some carpet along the bottom few inches. Grasp the bottom in a couple of places and pull (it is velcro'ed in place) towards you, in other words, towards the rear of the coach. Then wiggle it around some, including left to right, and it'll come off in your hands.
The next panel to the left, towards the center console can be pryed back a little bit as it's rightmost, bottom corner is again velcro'ed in place. But, I don't think the whole piece is removable without tools.
After moving the above 'second panel' back just a bit, you can probably lay down on the stairs/floor and reach way back in there.
I found our XM coax all the way forward and directly centered. Unfortunately it was zip-tied in a bundle and I couldn't reach the zip-tie to cut it. I managed to wiggle enough of it free from the bundle to make the connection I needed though.
Our XM installation is one of those little XM units that is the size of a deck of playing cards. It is sitting on the dash, just under the bank of idiot lights.
While you're in there behind those panels, take a look at things. Ours has the Vansco panel with all the LED lights-eventually you'll want to know where this is; large lugs for plus and minus 12V for possible connection of ham radio, etc.; there is a connector (or 2) for the Cummins diagnostics port (a tech can hook his computer up to this port to download data from the engine and trans computers); etc.
Good luck, and come back often. By the way, what is your name(s)?
Mike and Mike (Michele)
2017 Winnebago Tour 42HD
Previously: 2004 Alpine 34 FDDS