1993 GMC Suburban 1993 Suburban No Heat

I asked this question before in Feb 09. Since I live in Florida it wasn't a big deal. Still no heat, here's what I've tried.

It has frt/rear heat, so I by-passed the rear hoses to frt only heat. While the hoses were unhooked to the frt core, I used my garden hose on the inlet side and had full pressure water out of the outlet hose, so I know the heater core isn't clogged. I replaced the thermostat with a correct 195 and the temp gage reads 195 and lowers as the thermostat opens/closes. The radiator is full, correct antifreeze mix. Now the problem. The outlet hose of the heater core runs to a metal tube which connects to the back of the engine by the distributor. If I put garden hose pressure on this metal tube it doesn't flow. I removed the thermostat from the engine and tried it again and still no flow. What exactly does this metal tube in the back of the engine do? Is it coolant that runs through the block back to the water pump? When I touch the heater hoses they are slightly warm due to engine bay heat, not coolant. The metal tube stays cold even after a trip to the store. I'm stumped.