What David said. And what Doug said. Take the car to a vehicular HVAC specialist, have them pressure-test and leak-test the system. If the HVAC checks out, it might be worth also having a pressure test performed on the cooling system on the car. If the car has been losing coolant, it's possible that the heater core is depleted, which affects the ability of the heater to put out heat. That means identifying the source of the leak and replacing any failed components. Trust me on this, having owned several BMWs over my lifetime (E46s, E53s, E70s) none of them seem to have particularly robust cooling systems, and there are a seemingly endless source of leak points and fragile plastic nipples and hose couplers and o-rings, all of which seem particularly susceptible to heat-related decay.

Clear fluid can be AC fluid, brake fluid, or engine coolant (which the BMW fluid looks clear in small enough quantities, since it's not the more common Green/Gold/Orange).

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2007 BMW X5 4.8i - Sport Package, Wife's Car

2001 BMW 325i - My Car.

2005 Ford Excursion Limited 4x4, 6.0 Powerstroke Diesel. My first "grown up" car.