I have my Sirius antenna run through the 3rd brake light and sealed w/ silicon.. it works fine for me. ~2.5 years now and I have zero problems w/ rust or leaks.
As for mounting the receiver, I would highly recommend getting a mount like Thomas did here
. We put a similar bracket in my GF's Mazda for her Sirius set and it's really great. Easily the best way to mount a PNP set I've ever seen. He said he got his bracket here
. Although I know they are available elsewhere as well..
Get yourself a cigarette style +12VDC outlet from radioshack. Solder it directly to the switched accessory line behind your bezel.. then plug the car kit cable you got w/ your set into that and zip-tie it out of the way in the dash. This way you don't have to hack up the plug from the car kit, but you get it permanantly wired at the same time.