Prius overheating on highway. Changed thermostat. Water pump is operational.

I have heat in the cabin. I can feel the hot water coming out of the smaller thermostat outlet. But only cold coolant in larger hose to radiator. I theorized the radiator was clogged and almost replaced it. Then had idea to disconnect it and force hose water through it. I dont know if I cleared it or it was already clear but water flows fine through it. However, now when I drive it, still no coolant goes to the radiator. It looks like it just goes back into the engine past the little brass overflow pipe that goes back to resorvoir. I just can't figure out why it goes through engine and out into the cabin heater but it wont go through radiator circuit. I'm constantly scared I'm gonna overheat it and do some real damage. And its getting hotter by the day, so running the cabin heat on high is not feasible. It's not thermostat, water pump, or radiator. Any ideas?
Thanks

 

Okay, maybe I can quell all the speculation.
I started car. Turned on cabin heat to HI. It blows super hot. Then I disconnect the water pump which stops pushing hot coolant through heat exchange. I plug the water pump back in and within a minute, cabin heater is full blast HOT again. So to me, that rules out water pump as faulty. It's pushing hot fluid out the small thermostat outlet, but none out of the large radiator hose. This led me to suspect clogged radiator. So I flushed out the radiator with modified garden hose blowing both ways. It flows unimpeded both directions. Even when I pinch the small hose outlet to increase pressure to radiator, it still does not flow. There MUST be something blocking the flow between the radiator and the engine but there are so many little hoses and brass pipes up there, I can't figure out what the flow is. and I can't find a proper flowchart or diagram showing all the parts involved. If the cabin heater returns to the engine at a different location than the radiator then maybe that would help me diagnose. My next experiment is to run the garden hose into the radiator return hole and see if its blocked in the engine somehow. Maybe there's some extra thermostat or valve that isn't allowing the radiator fluid to enter. Worse case scenario, I can rig up the radiator output to Y into the heat exchange return because that is obviously working.
That help out any questions?
Ideas?
Thanks

 

Maybe I'll try thermostat on the stove idea. I did learn today, though, that the coolant flows INTO the thermostat and the water pump 'pulls' it from the radiator & other systems. Then it 'pushes' the coolant through the engine. I had it backwards in my mind. So I disconnected the lower radiator hose from thermostat and shoved my modified garden hose into the big thermostat hole. It pushed water all the way through the thermostat and water pump. Then through the engine and out the other side, then through the radiator, finally back out the lower radiator hose I had disconnected. The engine was still a little warm and so I was able to feel warm water coming through the radiator and hoses and out the other end. Then after a minute, the water cooled down as the engine did. Just sucks I cant drive around with my garden hose. :-/