so is one hose hotter than the other on your radiator? It sure is.... that scenario is the same for the heater core. Hot coolant flows in, heat is transfered from the core, and the return flow is cooler..... does this not make sense???? there should be a notable difference in the temp of the hoses. IF they both feel the same you have a flow issue.... At this point I'd try flushing/reverse flushing the core several times. I personally think you are going to put a heater core in. Your fan theory backs that up. Fan on low you get "some" heat. Fan on high you get none. Plugged core has no/very little heat to transfer.