Terk makes a mini-antenna, I believe the p/n is sir-mini and also makes a y-connection to convert a two wire to single wire antenna. In order to fit the Y-connection to the Chrysler Sirius box, take out a small tab or two on the connector with a Dremel tool. I recall I only did it to one of the two.
The mini-antenna is semi-gloss black and has a magnetic base. It's about 2"X3"X 3/4". You can install it on your dash under the windshield according to others but when I did, my signal strength was not as good as I thought it should be. I have mine installed temporarily on my console box door and it works ok even with the top up. Circuit City, Crutchfield, Amazon, etc. will have the Terk parts and they were fairly inexpensive if I recall($50-$75? for both). I did not use a Chrysler mounting kit at all, the Sirius kit was for the PT Cruiser, it's what they had.
My only complaint is I ended up having to stuff a bunch of thick, sticky cable that was provided for the PT and not needed for the Viper. There is probably a better Sirius kit with a shorter cable, but the guys at the local Dodge dealership only had this one in stock to let me try then buy. Please note, do not attempt to shorten/splice any cables to get the length correct. I called quite a few techs and was always told the same thing, signal loss.
I just did the Polk Audio speaker upgrade. The whole RB-1, Sirius, Polk works great. Good luck with yours.