If it has a "heat valve", check the cable going to it to see if its pulling open enough - IF your system has that. (I don't know much about 58s). Some vehicles do, some don't. If so, maybe it isn't opening all the way, but just enough to let water from the garden hose through.
If there isn't much hot water going through the heater core, the air blasting through won't pick up much heat so it'll seem cold.
Happened to me once, but on a non-GM. Hope this helps.
