99' Windstar tail light problem.

A message on my instrument info center came on stating that the left tail light is not working. This happened 8 months ago with the right
side, so I changed the bulb and problem solved. This time I replaced the left bulb and it still is not working. The brake light works fine, but no tail light. I went and bought another bulb just in case it was defective, but I got the same results, no tail light, only brake light.
Can someone advise me on what I should do/try next.
Thanks in advance to those that help.
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