Thanks for all your replies, it was mainly for 7D11 plus 100-400 lens, my niece has a farm up on the fells in the Ribble valley , the wind and rain blows some thing terrible, parked along there woods, would see me keeping warm and dry at this time of year .
To be honest, I can't think of a more awkward way to take photos.
Having to twist sideways to take photos, windows misting up, starting the engine because the window is open and now it is cold, the wind being wrong and the rain blowing in, coffee spilling, crumbs dropping between the seats
Then Murphy's law states that once you have everything comfortable in the car, something really good will appear in a direction not covered by your limited view
Plus in brings back memories of a game park in Rhodesia, with elephants making false charges at the car, and my father sitting on the drivers door window sill, with his feet on the drivers seat, using the car roof as a bench to sort out the batteries in the Canonet 8 not making good contact !
I would prefer some heavy weather gear and a camera "raincoat" for taking photos, and the car as a rest/break place.
To me, it is one of those ideas that looks good until you get to try it.