My left rear door window just went thunk and fell down, how do I repair?

More then likely your regulator has failed, you will have to remove your door skin and the window regulator. This may require several tools you may not have, and would be easier to just have done by a shop. Check eBay, or other online sites for a replacement most of the time you can get a motor and regulator combination a lot cheaper there, many with free shipping. This will save you a lot of money from OEM part and I have had very good luck with them.