We are trying to replace the broken rear left.

You can't take the cover off. The light comes as an assembly.
Open the tailgate and if necessary, pull the interior traim away and you would be able to see nuts holding the taillight. Remove the nuts and gently pry taillight off.
When installing, ensure the gasket is seated evenly and do not overtorque, it can result in the studs breaking away from the taillight.