When I was looking into a similar problem, I found a long thread in sci.chem about gluing PE/PP plastic. Search on "plastic welding", I think. Short version: it's hard to do. Someone said that Loctite has a product, "Plastix", consisting of "a surface activator and compatible cyanoacrylate adhesive," which claims to be suitable "even for PE/PP."
I did the soldering iron thing, to fix an inner fender and air intake ducting on a Cavalier. It's been holding for a couple of years.
G