I don't know about your cars, but a lot of cars have a location similar to the one in the photos below that is PERFECT for a pointing stick or integrated mini-trackball controller. The pointing device could be located on top of the handle, and the buttons could be located under the handle to be triggered with the fingers as demonstrated. I took these on my passenger's side for you UK'ers. My driver's side has a mirror switch as shown that would have to be relocated to make room for a pointing stick (though both devices could still be in the same space)

This idea could easily work for right-handed UK folks or left handed US folks providing your car has a suitable location.

The mirror switch on my drivers side door handle (left):


The blank on the passenger's side (right):


The hand position to use a device mounted here: