I just an 06 Trailblazer and since I already have Sirius, is there any possible way to use the current XM antenna to hook up a Stilletto 2 too? Since my old car had the Sirius built in, I just got a Stilletto 2 and thought it would be nice to be able to connect to the current XM antenna.

would this work?

http://www.tss-radio.com/sirius-antenna-adapter-p-3883.html

If not, I'll just use the antenna that comes with the car kit.

You can use that to connect to the Stilletto 2 car kit, but not directly to the unit. I use something similar to connect the factory XM antenna to a Sportster car kit. There is a guy over at SiriusBackstage.com that makes custom length adapters also.

No. Different frequencies.

No. Different frequencies.
XM and Sirius use different frequencies? Yes, but they're adjacent to each other. The systems might be different, but the frequencies are certainly close enough. The differences are in how the radio "talks" to the antenna.

So I guess the question is, are the radios looking for a "signature" that is specific to only XM or only Sirius, or do the radios care?

Read my post above....I have my GM factory XM antenna connected to my Sirius Sportster and it works just fine. You lose a little signal strength, but not enough to be noticeable.

Bottom line is YES, you can connect an XM antenna to a Sirius radio, the connector is just different, and there are a couple sources for adapters, including the one from TSS-radio.