After few busy days, finally got the time to try fixing the problem again. Thanks to SHU-ES1 about the cog. Apparently found out that there is a wire blocking the cog movement from stage 4->5.
Tidied up the wire, tested again. Foot vent is completely off now. YAY.
That wire is the Stereo Head unit's wire installed few months back. Now if i recall back a bit, the airflow vent problem came up after I had my buddy upgraded my headunit. It all makes sense now. =]

It is good to have it fixed, now i can enjoy my aircon better.
Many thanks again to SHU-ES1.