Yeah, my roof has got a few of those as well. I'll look for the pics when you finish! Good luck!
In case the inerior antenna doesnt work out, you can use a CB antenna as an FM radio antenna using one of these. You just can't use both at the same time (I think). I've heard it works okay.
Try these, I have heard to replace them with a cb and you still get radio. That or maybe look into a toolbox mount or the like