Just FWIW, the fin on the XM antenna is for Onstar (all model years).
Mine did not have the XM feature so the Onstar antenna only had two wires in it and not the XM part. I never plan to use onstar so this weekend I hollowed out the antenna on the trunk and took apart the XM magnetic antenna and put it in the carcass of the Onstar body. Works great. Took about three hours because I get distracted a few times but now nobody can see my XM antenna.
So here’s my situation just bought my 06 solstice, love this thing btw. Previous owner replaced factory monsoon head unit with a nice touch screen radio however I want xm and mine came with it from the factory but the new head unit isn’t xm capable don’t have unlimited data so the xm app is out. Xm sent me a onyx ez for free but I don’t want two black antennas on my car do they make an extender that used the factory XM antenna that I can just route and plug into handheld xm receiver?