In my little 5 door hatchback i have used a neodynium magnet to mount and space out rear cameras from the metal frame of the rear hatch.
My car are so cheap it dont have any form of plastic trim on the inside of the rear hatch, if you have plastic there you might be able to use the supplied dual sided tape to mount off that.
But in some cases you will have to find another 3M sticky pad to use as the supplied ones are for on glass mounting and so might not work well on the plastic trim, but try the original stuff, just know it might fail down the line and you have to pick up a few feet of a better suited 3M ( or similar ) product.
For me the #1 with a rear camera is to get it to mount so the lens are a hair from the glass, that way i dont have any heater lines in the footage.
PS. My Left side door camera mounted off the headliner behind A pillar, but also with neodynium magnet sticking to sheet metal foldet over and slid onto headliner edge, never had any problem staying in place ( lens also close to glass ) and the window down + 80 km/h highway speeds.
Have a look in here for ideas, start at the back at the thread ( newest stuff ) and work your way back in post / time.
As i recall my updated pics are somewhere in the 50 pages.
3D wedges repositioned for side-by-side Mobius testing:
dashcamtalk.com