Headrests won't fit! Help!!!

Hey everyone, I just purchased some Invision REV2 headrest monitors for my 2007 Tahoe. I was told they would fit my vehicle, however, during the install tonight I found the posts on the replacements were to thick for my seats! I tried to bore out the plastic cap that was around the original post but that only revealed a metal tube in the seat that is still to narrow for the replacements? Any ideas? I'm wondering if I was sent the wrong headrests... If anyone might have any ideas on how to make these fit I would love to hear them! Thanks for all your responses.

send them back and have them replace with the correct ones....I mean **** almost all new GM vehicles use the same size posts that are the same width apart....I have an 07 Esky which uses the same posts as my 07 STS, 07 and 08 Silverado, and 07 Yukon Denali.