i remember back, way back, when we used to drill huge holes like that in cars to mount antenas for "car phones". (yes there was once mobil car phones mounted in a car). that might be one that got removed and plugged. i have an 89 waggy that still has the wireing and window antena for a car mobil phone. that would be the optimum spot for the antena install back then, since you can get access from the interior light. you should have seen us drill these holes in brand new mercedes and all sorts of high end cars. im still kinda laughing about it.