A lot of us have aftermarket headunits. I have Pioneer but adding XM tuner should be the same on every other headunit.
Here is my
Pioneer FHP 8000BT
I bought refurbished Pioneer GEX-P920XM tuner from Ebay for around $50 shipped.

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It is looks like a big tuner and in most of the cars you would need to put it somewhere under the seat, not in the SMART!

In the SMART it easily fits behind the headunit!

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After you connected XM tuner and Headunit with IP BUS cable, you will need to connect GROUND and 12V cable.
Here is good locations:

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Here is location for XM Antenna I found and it is working for me very well.
I installed antenna in the stock tweeter location under the dash on the passenger side. No glue, no magnet, no tape, it is just slides in there like that spot was made for XM antenna.

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DO NOT FORGET TO DISCONNECT NEGATIVE CABLE FROM THE BATTERY BEFORE INSTALLATION!
It took me less then 30 minutes to install.
Now there is the best part! As I already mention the unit I bought is refurbished. So to my surprise after I installed it, tuner was already activated. I believe that unit was activated before and XM did not deactivated this unit after it was returned to the Pioneer. I do not know how long it will last, it might be 1 month, 6 month, one year, or lifetime depending on subscription.
If you have any questions, feel free to ask.
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Thank you everyone who already voted!
CUBE