Update with pictures
Over 40 people viewed this thread, so I thought I'd better update it with photos while I had the camera out and was replacing a turn-light.
My lovely assistant is holding the corner piece, which (for all its size) only holds a single small signal light for warning oncoming drivers.
This piece is held on by a hook at one end and a bolt into its backside through the front frame at the other end.
The part is susceptable to getting broken, because of location, and mine on the passenger side had been broken loose previously and was inadequately repaired (it rattled loosely).
Here is a closeup of the (unbroken) back-piece, which has a metal clip that slides over the plastic support and provides something for the bolt to screw into.
Here is a look at my repair-job on the passenger-side unit:
The plastic top-plate had broken off near the bolt. This plastic piece was kept, to hold the metal clip that the bolt actually screws into (it has a kind of simple thread that matches the bolt).
But to secure the broken piece, I decided to cover it with a piece of galvanized sheetmetal I had lying around. This arrangement is virtually unbreakable, but required a bit of fiddling with tin snips, and a hammer and pliers to shape the piece.
Here you can see why I made two simple cuts and left the metal sticking out. The other parts are wrapped around the original plastic piece, and the natural slot created keeps the metal clip securely in its place.
The left and right units provide a handy "before and after" shot:
This method of repairing the unit makes the best use of the broken parts, and the strongest repair, as well as the easiest. Even epoxy glue is not a reliable alternative in a situation like this. You don't want a piece like this bouncing down the road at 60 mph because the glue failed.
in hindsight, I could have made this even stronger by using an extended piece of sheetmetal, and placing the rivets further away from the original break.
If the other one is ever knocked out in the parking lot, I will make a larger bracket this time.