1969 Chevy C-10 Stepside rear tail light issue

[elektrik]

Hi all. First of all, I'm new here. Got a 69 stepside and all the directionals work-sort of. After replacing all the bulbs (the front end was being worked on) and verifying I had the *correct* bulb, when I turn on the directionals, the front ones work as expected, but the back ones work as if I turned the hazards (ex. I turn the left one on and BOTH blink!)..

Anyone got any ideas? Like I said, I know the bulbs are correct..

Thanks in advance