How do you glue these back on?

Most of those automotive plastics were ABS so use MEK to glue, if you need to fill, use ABS cement from a plumbing store, it's usually a thick gooey grey semi liquid melted ABS, I use it to build up around studs that have broken off making it stronger than OE when dried, and you can sand to shape if needed as it's essentially the same material you're trying to fix.