Heater Blows Cold Air

I need to try to back flush the heater on my 1993 Deville but cannot get to the hoses attached to the heater connections at the firewall without removing the A/C Accumulator. I need a procedure or a hose diagram showing the routing of the heater hoses from the firewall to the engine showing all connections. The heater return hose
is routed near the firewall relay cover and is accessible but it has a "T" connector with one hose going to the front of the engine and the other going to the rear which I believe attaches to the Thermostat housing. I cannot locate the heater input hose going to the engine or radiator. I believe it may be the metal tube that runs under the engine and is spliced about midway with a rubber hose and clamps. The hose is approximately four inches in length. The heater was replaced about 10 years ago. The controls to the blend doors are operating smoothly to the heat and cool maximun positions.