Heater doesn't work so good - 74 Duster
Heater core flow test
Grab both heater hoses with engine running and at operating temp. they should be roughly the same temp. One cool, one hot-core is plugged.
To flush core, pull both hoses. using a 2-3 foot hose on one side and another in a jug to catch what blows out.
pour a couple cups of water in hose and follow with a blast of compressed air. twice in each direction. the jug should be full of debris.