Air recirculation - where is the 'intake' vent?

[BigiusDickius]

I'm needing to clean the ventilation system in the car...got a cleaner spray but it needs positioned at the intake vent when air is being recirculated. Where is it and which setting should the air be going? Central vents or floor?

Thanks.

[polar]

Reserculate and open all vents put setting on windscreen and feet, turn on aircon and turn blower up full.

[BigiusDickius]

Would that not blow the spray away from the vent? Apparently it goes in footwell, so if air is blowing out it won't such in (if they do the blow and suck!), also says air con off...I'd have thought on as well...will give it a bash, ta.

[polar]

Will blow but as its on recirculate it will just keep going round and round the cab and ventilation system making it nice and fresh again.