yep the culprit is heater, I had same problem with my santro.
heater was leaked and winter came near, i went to mechanic and he said me its a half day job and gave estimation of RS 5000.
I came home, dismantled dashboard by myself, cleaned every bit of it which took entire day, took out heater, took it for Repair which costed RS 200. Fixed back everything neat and clean.
Job done with complete satisfaction.