A third brake light antenna adapter includes an antenna base attached to a spacer that is installed between the vehicle third brake light and the third brake light opening. The spacer is provided with gaskets, substantially identical to the OEM third brake light gaskets, which provide weatherproof seals between the spacer and the vehicle and between the spacer and the third brake light. Special support brackets enable the spacer to grip the inside of the vehicle passenger compartment to hold it securely to the vehicle without extensive modifications to the vehicle structure itself.