Use them on the Ford '26-'27 Stop tail lamp. The stop lamp bulb is std 21 cp. as the LED bulb is directional only, the LED array shines in only one plane. The tail lamp is LED, but this style also has an LED array around the side too so that a can cast a beam on the license plate. The light is bright but shines straight out as LED array does.
So you have to have a tail lamp socket that has the socket pointing straight out....not upright, as the LED array would then shine up inside the tail lamp housing. Not many tail lamp housings have the socket horizontal, most all are vertical sockets for the std 6v pin base bulbs.