I don't believe there is a separate antennae for the blue tooth. I believe the blue tooth is all on chip. I have 2 other mother boards in my house (both sandy bridge 1155 socket high end boards. )And they make USB blue tooth modules that are TINY. I think because blue tooth is only designed to be 30 ft that they don't have a big antennae on it. My expectation is that there are two because of the Intel WiDi to improve the reception of the standard WiFi. If you look on the card it actually states Main and Aux on them. I could be wrong. My laptop has a very similar setup and it has blue tooth as well. But of course nothing is external. Anything I have read states that the two are both WiFi and are to make better reception so I guess in theory My idea should work as long as the roof mounted one is as good or better than the ones included with the board.
The roof mounted unit I am talking about is the one in the mp3car store:
Anyhow I am going to finish up reading your build and I have to start a build log of my own it looks like.
You may be right about that as I haven't really looked into it. I had a WIFI Router about 2m away and disconnecting one aerial killed the signal, disconnecting the other made no difference so I just assumed one was BT - could be the way the router is running. Let me know what you find.
EDIT: BTW the roof top aerial in your link has 3 connectors - something like this may be my next purchase.