GTP Cooling System Flush

I want to flush my cooling system but can figure out which heater hose to install the flush tee. There are two heater hoses that exit the firewall (upper and lower). Not sure which is the heater inlet hose. Any guidance would be appreaciated.

lower one, but make sure that when you flush it you DO NOT turn the water on to fast

Thanks. We'll see how it goes. Pontiac dealer supposedly flushed & filled (with green stuff) a couple of months ago. I don't think they did a good job because it looks like they left some of the Dexcool behind. Antifreeze looks dark brown so I'm going to do it again myself. At least I'll know it's done right.