You can also buy this mount adapter plate to cover up your old antenna's hole pattern on the roof. That way, you don't need the mountain of caulking as shown in the photo in my previous post:
The thing I don't like about the adapter plate is you have to assemble it prior to mounting on the roof, so if you ever have to remove the antenna (base) for maintenance, you would have to remove the adapter plate as well.
I suppose though it is no worse than directly mounting the base on the roof OEM style.
The adapter plate is plastic. $30 is a lot of money for a piece of plastic.
When I custom build my plate, it will be wood encapsulated in a fiberglass skin. And I will use blind nuts in the wood so I can remove the antenna without having to take the plate off.