I am thinking of getting an Omnifi mp3 player, so I would need to get an AUX input cable. However, I am also planning to get Sirius Satellite radio too. Is it possible to have both? How would that work?

side-question: can you tap into the useless tape-deck for one of the sources? That would be perfect!

Jaak is still working on this idea as well... Please check this thread:
https://g35driver.com/forums/audio-video-electronics/8945-aux-using-satellite-harness.html

There is some new info towards the end of the thread.

From the other thread, it looks like you can have both, but have to splice into the system. The DIY guide is here: http://www.tvrules.net/misc/satpb/satpb.htm

Another option would be to use the PAC adapter and plug both the Aux & aftermarket SAT into that, but then you'd lose the SAT integration. Grrrr