At the risk of being the bearer of sad tidings, it sounds like the heater pump or valve. Could possibly be Fuse 15, 10A in the Engine Compartment Fuse box, or F3, 5A but that protects a few other things that you might have also noticed. It could also be the relay in the same fuse box - its the rearmost one of the three in a fore and aft line.
If none of them it could be the pump itself that's really awkward to get at on top of the transmission, by the main bulkhead. There's a useful guide here by RaceDiagnostics
Heater pump repair.