For the driver's side I mounted it on my cold-air intake box. Had it not been there I would have mounted it on the fender well next to the headlight. On the passenger side I mounted it on top of my windshield washer fluid box. I used some industrial-strength velcro I got from work to hold them on. Just make sure you clean the surface you apply it to first. Yeah deciding where to mount the ballasts was the hardest part.