Dug up a pic of the MR2 with it's old paint job. I tried cutting down some of the high points before I dip it in some paint stripper to prepare it for a better coat.

>>11496067
I guess to mount a mirror you could try laying masking tape behind the door panel and have a triangular corner peeking up where the mirror will actually mount. Use the tape to support the putty with mirror so that you can see if it looks right before you try applying any type of glue. Once its bonded, you can trim the excess tape/hardened glue with a razor knife and then paint over it.