The two different style gasket or seal base of the mirror caused the most stress for me. The wrong one was already on the mirror and the correct one was somewhere in the box.
Some people, I have heard, never got two, just one correct one.
Inside / under the top of the door skin where the studs go through, waiting for the nuts, MAY already have an area where the holes should be drilled. If you can, look closely to see. If I remember correctly, my holes may have been off slightly but it all worked well.
It is a tight confined space and my fat fingers were a disadvantage trying to start the nuts, that were metric, I think.
Enjoy!