Hi,
I've taken my dash off to sort my heater cables this weekend as my blowers have been stuck on hot ever since I bought the van. None of the levers seemed to do anything and it felt like I was forcing them, so I decided to strip it all down to have a look.
Turns out I had exactly the same problem as you. Rather than pushing the cable inner through the outer, the inners were just bending at the lever ends. Yours might not be installed incorrectly.
The root of my problem was the swivel on the heater valve. It had seized up and the cable couldn't push it. I lubricated the swivel and dribbled some 3 in 1 oil down the outers and worked it all free with my hands. Then I straightened out the bent cable ends in the cab with a pair of pliers and it all seems good now.
I was also missing a clip on the heater box, the ones that hold the rod for the flaps (
https://www.brickwerks.co.uk/t3-parts/t ... -left.html" onclick="window.open(this.href);return false;). The rod had dropped on the end with the missing clip, meaning the flaps inside were fouling and causing that cable to bend at the lever too.
I can get some photos of mine if you like?