He serviced the inner vents by disintegrating the unit once he took it out of the car (tedious task, requires atleast half a day). Also, I would suggest you to get your radiator and it's water lines flushed/washed, in which a pressurized water inlet was placed such as allowing the water to run through water lines/channels inside the radiator and the engine till the color of water coming out from the other end looked like normal water (not containing rust particles). I got it done right after the AC unit service, just so my car wouldn't heat up. Afterwards, I resorted to using radiator coolant/anti-freeze only, these fluids have a secondary function of rust prevention. It's a relatively simple task, takes 5-10 minutes. I got it done from the Shell Service Station near PIDC, right next to Rangers HQ.
While I understand your concern, you might want to take a moment and reflect upon the fact that everyone here shares his/her suggestions based on experiences only, considering it should work for you since it worked for them. However, there's no guarantee that your case is identical to other's. We are just suggesting things based on what you've explained. The accuracy of your description along with my understanding is relative. Since, obviously, we're not gods here.