You need to make sure if you are mounting a camera on the exterior of the car that the suction cup is clean, and the surface using is clean as well. Dirt and debris make a suction cup not stick. A quick wet wipe of the surface and suction cup means that as long as you are getting an air tight seal, the suction mount should stay on. Glass being one of the best surfaces because it is flat. I use Go Pro and their suction out for exterior shots. And I have a mic close to the exhaust which is held by electrical tape.
Interior shots, easy. Just get a suction mount for your phone. An iPhone with an interior suction cup mount even off Amazon works just fine. Attach to any interior glass surface and you are good.