Who has changed their factory radio?

I am trying to get the Pioneer cd player mounted in my 08 Silverado,everything is going pretty straight forward I have all the adapters and mount kit etc but I cant get the antenna to plug in.The big black plastic deal on the end of the antenna cable wont allow the adapter to mate the 2 together.Do I have to cut this off? I supposedly have the right adapter it came with the PAC interface for the Onstar feature,everything else plugs in fine just the antenna.

This isn't the first stereo swap I've done so,every other truck I've owned I have changed the radio in this is the first I've had to deal with all these interface modules for Onstar and such.