This is a problem beginning 2 years ago when temperature is in 40s and 30s.
I must drive 12-15 miles before the heater produces warm air. What is the proble on 2004 Toyota Highlander
Just a thought; Try putting a piece of cardboard or ? in front of your radiator to block some of the cold air, (like 18 wheelers do). Or change your thermostat.
I agree with ProfessorG. Replace the thermostat and gasket with Toyota OEM parts. call a dealership for the part numbers, and check ebay.com for better prices!