I don't know much on A/C darts but I would take the valve out. That stops the heat in A/C mode. Then you'll have heat possibly in A/C mode. Also plug hose so no vac. leak.
Ok, I was thinking that would help too. But still curious why the fan only works real low on heat, when it worms great and blows hard on ac mode? I live in oregon and it rains, and having a working, hard blowing defrost is something ive got to have.