At about 100km/h my drivers door is vibrating like crazy, especially around the upper window frame. I can stick a finger between the car body and window frame when its doing this. I tried to adjust the door striker but there doesn't appear to be an adjustment. The hinges don't seem loose, nothing obvious appears to be wrong. Any suggestions? This doesn't seem to be a common problem, I haven't found any other threads anywhere on this particular problem.

 

If this is a new issue, it seems that:
1) your door window frame has been bent or
2) the door seal in the window frame area has been damaged.

Did someone try to break into your car by prying on the window frame?
Do you (or someone you love) like to lean on the open door?

 

It's not a new issue, the car has done it since I bought it two years ago. It just seems to be getting worse, or I'm imagining things. I suspect it was opened by force along the line. I saw some videos on YouTube about bending the window frame back in towards the car, I'll give that a shot.

 

To be honest I've never heard about this problem in a Fit.
I think before you try bending the window frame I would make sure the door has the rubber door seal attached to the door opening and working.
Compare the car's front two doors for the factory door seals and make sure the seals are present on both sides.
You could also get some advice from a local body shop to you about the cause of the vibration.

 

If you try bending the door, don't forget to do it with the window open!

 

Gave it a little bend, worked perfectly!