For a solution that doesn't require you to cut the wires on your Xenarc, you can buy molex Y-adaptors that will split the output of the 12v wire inside the PC downstream of the M2-ATX. Then, you buy a molex connector that plugs into that and ends with bare wires. You buy a FEMALE cigarette lighter connector and wire that to the bare wires coming out of the molex connector.
Then, plug the Y-adaptor into your harness and connect one side as normal. On the other side plug the molex to female cigarette lighter adaptor you just built. Plug the Xenarc into the female cigarette lighter and you'll have power for it.
If you don't mind cutting the wires on your Xenarc (most people end up doing this if they have a permanent installation) you can CUT the wires going to the Xenarc and connect the molex connector right to the wires and plug it directly into the Y-adaptor. That skips the step of putting the female cigarette lighter plug in the wiring but it requires that you cut the wires.
No cut solution is Y-adaptor --> molex connector --> female cigarette lighter socket --> Xenarc cigarette plug
Cut solution is Y-adaptor --> molex connector wired to Xenarc power wires
Oh, and radio harness is not used. Only thing you might need is to run an accessory wire (power on) from the radio harness to the M2-ATX to signal it to turn on and off. There are other locations you can get this signal from, including using a fuse tap inside the fusebox. Not sure where you were planning on gettting it from.