Mine is mounted in front, about three inches back of the windshield. If I am remembering right, I ran the wire under the dash and out the door. Then ran it under the windshield gasket up and around to the top middle. There is virtually no visible wire, other than a small amount in the door jam. I'll have to check, but i don't exactly remember how I got it out the door - whether it was under the trim or through the wiring harness that goes to the door speakers.
I have the Sportster. The other thing I did (in addition to using the aux port wiring rather than FM transmit) was to change the 12V plug for the task light under the glove box to a 12V outlet (cigarette lighter type). I plug the power supply into that, so my cig lighter on the dash is still available for other accessories. Unfortunately, you can't get too clean with the power and direct wire it, because the power plug is actually a transformer that steps the voltage down to 5V - so you pretty much need to use the plug supplied or get real creative.
I haven't had any issues with the signal (other than typical Sirius reception issues). But it works as well or better than my other cars.
I might be able to get some pics up this weekend. I strongly recommend using an aux port to connect to your stereo. That, or use the adapter to directly wire into your antenna cable. Relying on the FM transmitter is awful as you get static and station bleed over. Older Sirius radios had higher wattage transmitters so it wasn't a problem. But I think the FCC limited the power on newer ones because other vehicles were being effected. In fact, with my old Sportster, several of us could tune to the same station and listen to the satellite radio. Not anymore . . .