Originally Posted by Strife

I don't think it can be done without pulling out the headlight assy at least somewhat, not a big deal. I assume that you have US headlights. First, remove the headlight cover (the white plastic) with two philips screws. Then, there are screws holding in the (sheetmetal) headlight assy. Remove these, being aware that the assy will be loose when all are removed (don't drop it, on your bumper cover, for example). I always put a towel over the area. At this point, you might want to disconnect the wiring from the car body to assy at the connector (wedge a screwdriver between to get it started). The side marker nuts (3?) should be visible. Remove these noting any nylon spacers, lock washers, etc and their order. Remove the broken plastic and the gasket (you probably got a new gasket with the marker plastic, I'd use it). Note how it was installed.

(at this point, you might consider replacing the bulb if it's old)

Replace the gasket (correctly!) and then replace the plastic. Replace the hardware (lock washers if any, spacers, etc). Tighten enough to seal the plastic to the gasket properly, but do not overtighten, because you could break the plastic at the "stalks". Continue reassembly. Do not overtighten the front plastic door screws, either.

When reassembled, it is unlikely that you disturbed the headlight alignment (much), but it should be rechecked.