Two places to look: the switch and the resistors. The switch, depending on its position, puts current out in the brown, the green, or both wires. You can check that with an ohmmeter. The resistors are in the heater box where a T-shaped connector plugs in behind the glove box. I think there are two resistors in there. Check them and the connections between them.
If I were to guess, I'd bet on the switch. If it is bad you can carefully remove the back and clean the contacts with some 600 grit paper and put it back together again.
Good luck.