Have one....
Link:
http://www.gumcreekcustoms.com/Universal-Vehicle-Handgun-Holster-Mount-p5.html
Added this: It's a "mount" not a holster. The mount supposedly connects to most handgun holsters.
and it works very well. Since I don't have much padding (6'2" 180#) this helps relieve my back of an imprint when driving. I don't always need it as my carry method varies.
I attached a basic nylon holster to it and it works. The soft holster is a good choice to save my knees when entering and exiting the vehicle. Be aware that you need enough room for the mount and the gun to clear your body parts. Not really going to work well in a sub-compact (car) situation. Unless you have very short legs. If I have time, I'll try and get a pic this weekend.
Overall, a well made product.
You can see the attachement hooks. They key is that they have to be pulled tight. If not, when you remove the gun from the holster it will tension to one side and drop the lower hook loose from the vehicle. Therefore you need to have it good and tight.
I am looking at adapting this to attach to drivers side of the center console (just in front of the drivers seat). I can't make any holes in the vehicle, so it needs to be a 'neat and clean' install. I think it will be a summer project.