I'm putting one of those hidden antenna's in my car. My question is how should I plug the hole from the old antenna in the back?

Cut, weld, sand and paint. Anything else will obviously not look right.

Wine bottle cork LOL.. just kidding... Unity is in the right ball park..... what else can you do...
Bondo??? if that's what you want.

unity's got it right...if you bondo,be ready to do it again soon,real soon. do it right the first time and there will be no second time...imho

Alright thanks for the advice guys. Hopefully I can get it going soon and get some pictures up.