98 Jimmy No Heat

I have a 1998 jimmy and I have no heat when she is at idle, but I can get some warm air when driving (warm not hot) I checked the rad. and it was full.
Also when I try to turn on the A/C the light on the switch blinks 3 times and goes off, (it was a fiarly warm day when I tried this in the 50's)
Just wondering if the heater and the A/C were intergrated some how.
Also the top heater hose on the firewall was warm but the lower one was not (heater core?)
I have noticed I have the red junk (dexacool er something) in the rad.
does this cause problems?
I had the steeler.,.er dealer.,., flush it a long time ago like in the spring.,.,.
I have changed the thermostat and the truck does NOT overheat it pegs out @ 195 er so.,.,.
I have read back flushing the heater core may fix this problem, and also read that the overflow tank hose needs to be clear and tank with coolent in it (have not checked this out yet as my wife took it to work today)
dang it I told her I wanted to work on it today, so now I have time to research,
any Ideas~? I don't want to replace the heater core if I don't have to as it is not leaking.
Thanks~!
vas002

Yeah, first thing I would look at is the heater core, disconnect it and back flush it. It should flow good. When your done flushing the heater core, fill it up, and purge the system of any air. If the top intake hose for the heater core is hot and the return line is cold, chances are you need to purge the heater core and get it flowing again.