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If I am not mistaken, the bulbs plug into a socket that has built in contacts. The socket is then pushed and turned into the housing. You will have to clean up both contacts, the socket and housing parts, and then bend up the contacts in the socket a bit. That should help you, I know it did for me.
To rule out any problems with your housing contacts or socket contacts, you will need a test light or volt meter. You can unplug the wiring going into the housing and have someone step on the brakes. If you have voltage, it should show or the test light will light up. Another place to check if you have a tow hitch on the back is the plug-ins under the vehicle.