The seam is from there initially being two pieces assembled together, but as you've discovered, they don't seperate easily, and my advice is based on that - they will likely be broken beyond repair if pried apart.
What exactly are you looking to do to the side mirrors that you want to disassemble them? Install puddle lights or side mirror turn signal arrows?
Getting one to take apart might get you some insight into how to take one apart to do as little damage as possible, but in my experience, original first gen mirrors are now 20+ years old, and the plastic is fragile and brittle and they don't come apart without damage that make the disassembly pointless, unless you just want to see what they look like inside. The mirrors are supposed to pop off as well, to make replacement easy, but it seems the plastic mounts / pivot breaks instead of just the mirror popping off the mount, which, again, damages the assembly beyond basic repair.