The best placement is the center of the roof because the antenna uses the roof of the vehicle as a ground plane. You can make other locations work but the middle of the roof gives the best reception and transmission of the signal from all directions. Mounting it to the rear, like on a third brake light mount, creates lobes with the strongest to the front of the vehicle. Mounting it on the side of the roof creates the strongest lobe toward the opposite side of the vehicle depending on where it is mounted. You can still transmit and receive from other directions but not as well as from where the main lobe is pointed.
For me, since I have roof racks, I can't use the center of the roof. So, all of my antennas are in compromised locations that don't give the best performance. The only one that is optimized is my CB antenna. It's on the right, rear corner of the truck so the main lobe is directed at on coming traffic. The rest work ok but since on are on the edge of the roof or the hood, they don't do as well as they could.