also you can go to home depot, superglue makes a special bonding agent for Hard plastics that are normally hard to glue togther. I dont have the info here but i might have it at home. I used it a few months ago, it worked great. its a little superglue tube of adhesive, and theres this little adhesion promoter thing too. the pastic that the headlight is made out of is one of those "hard to bond" plastics. not sure what it is, might be polypropelyne. but most glues say "bonds most plastics except bla bla bla" and 95% of the glues wont bond whatever the headlight is made of. I tried using the 2 part plastic weld from pepboys and it did nothing.
this will work
Plastic-Fuse - Super Glue Adhesive Products