I'm assuming you are talking about the third Brake light. I had some difficulty the first time as well - someone had run a little sealant around the edges of the gasket (both sides).
Your best access is raising the pop top - I was able to loosen mine by giving it a couple nudges from both sides at the same time. On mine, the interior wire and the metal piece which holds the bulb were all corroded, so I purchased a replacement unit through the SMB Store (now Field Van - not sure if they offer it anymore).
This Thread has some relatively recent replacement suggestions:
https://www.sportsmobileforum.com/fo...ssy-30963.html