Air not blowing out dash vents

Clean the battery cables. They may be making bad contact.
Starters usually do no go out all at once.

if both vents are blowing air in general then you may need to get your air conditioner recharged, but if the front vents aren't blowing at all then you may just have a jammed vent flap and you can fix that from under the hood next to the blower motor. check you rowners maual for the exact location

vw group products used to have a bleed valve on the top heater hose,to vent any trapped air,now you have to undo the jubilee clip on the top heater hose,ALL vw NEED to be bled after refilling water system.

check under the dash to make sure the vents are hooked up

Sounds like the vent control motor is bad. It controls how much hot water flows through the heater core. OR the heater core for that side is clogged

If it blows thru other vents, your air ducts may have come loose. If you don't get any cool air, your coolant is low (it evaporates during the time and below some point it will not circulate anymore). Also the aircon blower can be faulty.

Heater resistor faulty,located behind dash.You need a good cheap auto elec tech,not vw dealership.Good luck.

this is because you have a bad switch which has gone bad and you need to replace it or either try to bypass it through another circuit.

add coolant....if that doesnt work...check the thermostat...then if its not that its the water pump

The heater in your car depends upon a flow of hot coolant going through the heater core in order to get hot air into the car. If the heater core is partially plugged, it will restrict the flow. If you have a head gasket that is putting exhaust into the cooling system, it will eventually get into the heater core. When this happens there is no coolant in the heater core to make the air warm. If the water pump vanes have corroded away the result will be very poor coolant circulation causing the heater to not blow hot air. If your car is equipped with a heater control valve that is not functioning properly, the coolant flow will be restricted. One or a combination of these factors is likely the culprit in your case.
if the car is not overheating, the thermostat is working fine.... replace or flush the heater core multiple times to try and eliminate sediment build-up.