a lot of the polymers get very brittle in cold weather, so you might want to keep that in mind in the future considering you're in a cold climate. A lot of polymers can be welded with a soldering iron, if you can find another panel that has some excess material that you can trim a strip off of, and then using a soldering iron weld it to the backside of the panel spanning the break. Reason i say another panel in the truck as welding the same type polymer to the panel will be the most successful.
Not having seen that panel in person, if the front side is flat, cover a work table with some formica (or similiar) then you can solder or weld it with the table supporting it from moving - as you weld it, you'd want someone to press the plastic strip into the back of the panel. You might want to practice with a small pc of the plastic strip to see how much heat it needs to soften and weld to another pc of plastic. Some of that instant glue that comes in small tubes, a few spots of it on the edge of one panel and slide them together and hold until the glue has set, will serve to keep the break (or the gap) closed as much as possible while you're soldering.
another approach, if you have any experience with fiberglass, if you roughen the back side of the panels, you can bond fiberglass cloth to the panels, wetting the fiberglass with the correct epoxy. If you were to go that route, i'd wax the front side 2 or 3X, so that if any epoxy were to bleed or wick thru the crack it wouldn't get a good grip on the front surface and could be popped off
hope that helps