Locating the XM antenna inside OEM antenna base

(Undertaken 2/20/06)
I installed the XM Direct (XMD) unit that plugs into the pre-wired harness in the right rear quarter panel accessed from the boot.  The factory radio will accept Sirius and XM, but shows all satelite connections as Sirius, because that's what MINI offers from the factory.  I also have the controls on the steering wheel for channel and volume on the radio.

The XM antenna that comes with the XMD unit is about 2"x1.5" and reasonably small.  It comes with about 25 ft of wire and can be installed almost anywhere, but it's preferable to install it on a large flat body panel at least 12" square.  It has a magnet on the bottom to attach it to the body.  You can also use double sided tape to adhere it to non-ferrous surfaces such as the dash board I guess.  I initially installed the antenna puck behind the stock antenna base in front of the roof spoiler, which allows for a neat arrangement of the wire down the trim edge to the XMD unit.  My concern was mainly when I washed the car, as water would sit under the puck and leave a drying residue, or I'd have to remove the puck each wash, which leads to micro scratches of the roof paint.  There is a thread on NAM about installing the Terk antenna unit in the OEM base, so you should refer to that as well.

The photo below shows the base removed from the car and the XMD antenna unit installed.  You can see the antenna is quite small -- about 1" square -- and there's lots of room for it in the base.

The steps to install are pretty simple:
1. Pull down the headliner from the center.  You can be pretty aggressive with it.  If you have a sunroof you can access the antenna base through the plastic cover housing the motion detector inside the car.  But with a solid roof, you have to get access from the back opening under the liner.
2. You must get a pair of pliers or a wrench (18mm) into the cavity under the antenna base to loosen the nut holding the base on.  It's not super tight, but can't be loosened by hand, nor is there much room up inside the cavity (between the outer roof skin and the inner roof skin).  There are lots of wires up there, so be patient.  Once the nut is off you'll see two wires coming from the antenna connected via removable connectors to wires in a harness.  These are the radio antenna and another wire from the radio antenna circuit board pictured above.  Unclip these wires.
3. Lift the antenna base off the roof, bringing the two wires with it.
4. You can undo the 4 screws holding the antenna cap to the base, they are small Torx screws.  The plastic cover will now lift off, or you might have to pry it a little with fingers.
5. The base should like a lot like the photo above.  You'll immediately see where the factory Sirius antenna fits.
6. Remove the 4 screws from the XMD antenna and pull the antenna unit, which is primarily a circuit board, away until that's all you have left.
***Now here comes the hard part***
You cannot pass the circuit board through the hole in the stock antenna base, nor can you pass the connector at the other end of the wire through the hole (you can see how small it is in the photo above.)
7. You must cut the antenna wire !!!  Scary, as it's a true coaxial cable, with braided outer shield and insulated inner wire, which is incredibly fine, but not fragile.
8. I messed around for a long time trying to splice the old wire back together again after cutting it close to the circuit board, but the XMD unit kept giving error codes.   Finally I unsoldered the circuit board cover (it's a tin shield, on the bottom of the unit in the photo above.)  This revealed that the coaxial cable is soldered to the board inside the cover, with the shield soldered to the outside of the tin shield where the cable enters the antenna.  I removed the original (short) wire I had cut, prepped the cut (long) wire with the connector on it, passed it through the antenna base, then soldered the coax into the XMD antenna circuit board.  Then re-solder the tin shield back into place.
Sounds difficult, but I did it with little fear, a good soldering iron, magnifying glasses for close detail work, and experience working on circuit boards. 
9. I used double sided tape to mount the XMD antenna to the base as shown.
10.  Connect the antenna to the XMD unit and check that the radio works!
11.  Reassemble the stock antenna, thread the (now) 3 cables into the roof cavity, and screw the holding nut back into place.  This requires patience as it's very awkward to get hands, fingers and tools into that space up there.  Connect the original wires, then push the headliner back into place under the rubber edge seal.  Run the XMD antenna wire under the rubber seal and down into the rear right cavity where you have the XMD unit installed.

Done.  The antenna base will look something like this below, except you won't have a shark fin since I custom fabricated that.  I don't use the stock AM/FM radio, and the reception is fine without a long antenna anyway.  I'd never go back to broadcast radio anyway, after listening to XM.