So do you currently have a shark fin antenna? If so, the wire should be down where the receiver was. If that is the case, you'll just need to remove the plastic around the coax plug on the wire and plug it into your sirius receiver. The xm and sirius antennas are completely interchangeable and work with one another. Actually, both the antennas for both trucks are XM units, but I have sirius radios.
As far as putting another antenna under the sharkfin, forget about it. There isn't much room under the sharkfin really. You could do it, but if it still works why bother?
If your trying to add a GM antenna to your truck, same principle applies just as long as you get a single wire antenna. Any of them will work. If you need an antenna, there are a few styles floating around. The problem is here in the US, GM used a very large XM antenna in 03-04 and 05+ switched to the sharkfin antenna. The sharkfin is for the onstar portion, however some are just XM. I don't care for either of those antennas. I did find a very low profile GM XM antenna that was used in canada. I have one i plan to install in my 02. It's about 1" x 1.5" and only 1/2" tall at most. It's as small as most aftermarket magnetic units. I can get the p/n if your interested, I think it runs about $40-50 at the dealer.
