If all of the light bulbs are removed from the housing and any plastic ties attaching the tail light harness to the tail light are broken /cut / removed you should be able to replace the housing without replacing any harnesses. I can't remember exactly what it looked like when I changed my bulb but it didn't look too bad. I don't recall if any harness was actually tied to the housing or if it was just the 2 bulbs.
If you need to change the tail light harness with the housing the tail light harness is a relatively small pigtail with a single harness connector on one end, a grommet which seals the hole in the body work, which then splits into the individual light bulbs for the tail light itself.
If you need to get to where the tail light harness plugs into the main vehicle harness you need to get to the interior sheetmetal of the body behind the tail light (s). This is the only photo I have on hand from when I installed my trailer wiring harness. This is the interior panel removal necessary to get to the tail light / body harness connection.
Again, if you're not comfortable with all this its probably worth paying a shop for help. I can't imagine it would be pleasant breaking and replacing one of these panels, the harness, or the new housing.
I myself would free the existing harness from the existing broken harnesses and re-use it if possible, avoiding any of the interior panel removal.
If you can take a photo and identify exactly what you're saying is stuck and you can't remove someone may have better advice. A picture is worth 1000 words. I may be way off base what you're actually asking for.