These trucks (1994-2001/2002 dodge) don’t have the greatest HVAC systems. Replace the T-stat first, it’s easy to get to and a fairly cheap part. If that doesn’t solve the problem, use a funnel and hose and run some boiling water/cleaner mixture backwards through the heater core. This might dislodge whatever it is that’s clogging the core, and save you from pulling your dash out to replace the core.