A little dab of super glue and a steady hand. Be careful when using tweezers. Things can go 'zing' in a hurry when you least expect it.

Oh yeah… they have a habit of disappearing into the vast nothingness.

Honestly, I would have left the attachment on the sprue and glue the mirror onto them. Makes it easier to assemble. Once dried, you can nip them off the sprue and onto the car kit.