No flow to heater core
 

Hello, I have a 2004 Audi A4 1.8t Quattro and there's minimal to no flow to the heater core. I hooked up a garden hose directly to the heater core, let water flow at low pressure, and it has great flow. Do you think my thermostat isn't opening up to allow flow to the heater core? Or maybe there's something in the coolant line blocking flow? If I rev the engine about half throttle, I'll get SOME heat through my vents. Also, when flushing water through the core, there was a significant amount of rust flakes that came out. To me, that insinuates the water/coolant in the core was pretty stagnant. Your thoughts.........