Just to be sure, your talking about the plastic grill and not the fiberglass header panel right?
Some plastics will soften in acetone. By dropping some shavings into a container of acetone over night, you will end up with a glue to use for repairs. Here are a couple paragraphs on the subject that I pasted from another site.
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.
Start with 100% pure Acetone, buy a gallon.
Pour an ounce or so intp a glass jar (small open mouth jar). Take a scrap grill and using something sharp, scrape off peelings (like pencil shavings size) into the jar of acetone. Put allot of shavings in, the acetone will gradually melt the plastic into a goo. Add enough plastic until the goo reaches toothpaste consistancy. Once you have a good batch melted put the lid on the jar tightly and set it aside. Now, prep your grill as normal, V out cracks on both sides, etc. Use clamps near the joint to alightn the two sides flush and parralell. Smear on some plastic goo, leave it crowned up over the crack like a regular weld looks, let it cure overnight. Sand to flush the next day, fill any pinholes with JB Weld. The crack will practically dissapear at this point, paint as normal
