Attaching an XM radio

I have a 2012 Tacoma that came with the XM radio in the trucks radio. I didn't want to pay every month for that so I got a lifetime subscription to XM because I already have one for Sirius, so I got the vehicle kit, boombox, etc. so I can transfer the radio wherever I want it. I plan on running it through the AUX port. My question is this: since the truck already has that shark fin thing on the roof which I assume is the antenna for the XM radio is there a way to use that instead of the antenna that comes with the vehicle kit??(it would seem cleaner to use it since it's there) I was told on a chat with XM that I could not use it and to use the antenna that came with the kit. I'm not that crazy about tearing the dash apart but I figured I'd ask here as many of you are far more knowledgeable about these things than I am. Thank you!