I've got a '92 Runner with rear heat, which does work, but nothing out of the front, it's been sporadic to say the least. Checked the heater core for blockage, did the thermostat, checked the HVAC cables and operated on the heater valve mounted on the firewall. I'm assuming the last culprit would be the water pump, although I haven't had any overheating problems and there's rear heat... Any ideas?? Thanks.