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Ground problem. You have to check ground continuity at the bulb contact. You cannot just verify the ground wire is attached. Taillights are finnicky about the grounds. The lower voltage on one circuit is due to the higher resistance (due to a poor ground) in that circuit, so you have a lower voltage. Your poor ground may be in the socket, so you must check the ground circuit with an ohmmeter. Anything more than about 0 will be problematic. Good luck finding your ground problem!