Ronnie has 26+ Vegas and a few Monzas at his place. He has a website: http://www.vegaguy.com/
And has regular ads on Knoxville, TN Craigslist.
He took a few minutes to show us how he repairs the early Vega Valance panels.... the ones that are a hard-type plastic ('73 in this case).
He uses a butane powered soldering iron.
First, he places the two pieces together and simply "welds" the two pieces together with the soldering iron tip.... the one that looks like a flat-head screwdriver (Sorry that I don't have a pic for that process... my cell phone camera didn't work like I wanted it to so it got lost).
He says if you remove the paint first and clean up the area to be fused, it works even better. This was just a demo, so he simply "welded" the pieces together blending paint and grime all together. It was surprisingly strong.
Simply use a circular motion down the crack to "weld" (melt) the two halves together...... just like welding.
Second, he uses the spoon-type attachment to scrape the excess/buildup off... making the surface more smooth.
Third, he then uses the same attachment to melt some shrink-wrap tubing onto the repair to act as a filler material.
This valance was completed in less than 10 minutes..... obviously, more time would be used to clean/prep the area but since this was just a demo it wasn't a pristine job, but you get the idea.