The lights are controlled by either one or two vacuum relays, located in the middle of the two lights, IF you have just one relay, and one light works fine, check the hoses leading to the other light, if the hoses have proper vacuum on them, and it changes from one to the other, then the canister is a problem, now the canister has a vacuum seal around the actuating rod, the outer bellows is just for dust, inside is a thing called a 'witches hat'.....but that hat only affects the operation in one of the directions, I can't remember which...sorry....
if you have two vacuum relays, check that the control line to the relay is operational....you might have a bad relay even.....
:goodnight: