Crank,
Well, I'm not looking at an example of your car, so I'm not sure just how things should run, or how badly the heater interferes. I will say that the less these cables divert from straight, the better they operate. Are you sure you have the correct cable? with the correct length hard housing behind the dash? and located in the right opening?
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon