Okay I can see how that's confusing. XM and sirius have proprietary hookups, so you can't buy an OEM chrysler XM receiver and plug it into the mazda unit.
What I mean by an aftermarket unit is one that comes with a receiver, antenna, and seperate control unit with LCD. For instance the
sirius starmate.
All of these aftermarket units have built in FM transmitters, and audio outputs (usually a 3.5mm jack). If you got one of these you could try out the transmitter, and if you're unsatisfied with that then you could get an aux. input to your stereo. Look
HERE for a list of audio input options for the 3. You'd be looking at ~$60 at the cheapest, up to $150 or so for a direct input. The upside to one of these units is you can take it out of the car and use it somewhere else, connected to your home stereo for instance. That way you get more out of your subscription. The down side is you can't control it with the OEM HU controls, and you have to have this second HU control somewhere. I just helped a friend install a starmate in their 3 the other day. It fits perfectly in the open mouth of the ashtray compartment (not mounted, just wedged) and she can hide it inside, or disconnect it and remove it very easily. She's using the transmitter right now, and I was surprised at how clearly and well it worked.