I bought a used unit on ebay that has this problem. I am returning it because I don't want to deal with this, but I have found a workaround.
I'm not sure if this will work for everyone, but it worked for me. My problem was worse than some have described. My unit would reset at the splash screen, so early in the process that I could not ever boot into testmode. After much playing around, I discovered the following sequence that seems to work. Keep in mind that the slightest deviation from this exact sequence causes the procedure to fail.
1) Turn the car on, allow the unit to try an