If you have installed your left one already, then I assume you've got the adapters/ studs already..... but I know that the left and right side mirrors are threaded opposite of one another..... I cannot recall which one is actually backwards threaded, but make sure you're trying to screw it in the correct direction of the threads. (for example the left mirror may screw in clockwise where the right will screw in counter clockwise...)
If hat's not the cure for your installation woes, then maybe you could attach a couple pics to your post so we can see what you're working with.
