It's called MEK (Acetone works to). Mix up some in a glass jar with some shavings of ABS (old grille pieces work well) and let it sit overnight (it will melt into goo) you want to end up with a mix that's something roughly the consistancy of wet peanut butter. Prep the edges of both pieces of the part by sanding them to a beveled edge, point the edges together aligning them and use clear packing tape to secure them and keep them aligned from the underside. Smear a nice pile of the goo into the new joint on the top side until it's piled high like a weld in metal. Let it dry a couple days, sand smooth. For thicker parts do the same process on both sides repeating the process the same way.

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Here's an ancient Moparts thread as well Moparts Grill Repair