its been awhile since my punto was out and about, but from memory, the tail lights are screwed in place (as above posts) to affix them onto the body. then, the wiring is fitted via a firm plastic plug / socket arrangement. this should unplug (pull by plastic and not by wire!) enabling you to then refit onto the replacement light unit.
as a suggestion, if the replacement unit has come from a scrappy maybe the wires have been cut instead of unplugged - so you will need to unplug and dispose of the duff terminal plug?????