1999 Isuzu Rodeo Question Defrost activates air conditioning

This is typical of many cars. The A/C unit is meant to dry the air before it gets blown onto the windshield. Nothing worse than blowing damp air onto a frozen window. My advice is to only use it when needed, otherwise use the "heat". The alternative is to disconnect the power wires to the clutch so it cannot engage. And just plug it back in come summer. Ba dthing about that is even during the winter you are supposed to cycle your A/C at least once a month to keep it oiled.