Heater not real hot

I own a 1999 toyota rav4 that I purchased in the summer. The car is in great condition, but being the season it was, I didn't check the heat. As I'm driving the vehicle, it SLOWLY heats up, but when the car is just standing, or parked, the air cools down. I'm sure the heat is supposed to be running hot while the car is on, even though I'm not driving it. My older sister has a toyota and it gets real hot in her car so i know my heater isnt right. *I haven't told my dad yet, because I don't want him to think I broke it...or worse... Made a bad decision. He had to cosign for me.*
Is it going to be hard to fix without Dad finding out? *Could there be a thermostat that can be turned up? *It's hottest when the ac button and no defrost button or slider is on, but even then it is not very hot. I drive 20 minutes at 70 mph and it still doesn't get hot. Can anyone help me? *Thx!

No answers?

Have you checked the coolant level in the radiator overflow tank? There are marks to indicate what the coolant level should be when the engine is warmed up. Not having enough coolant can cause lack of adequate heat. It should be hottest when AC button is not on.

could also check the thermostat. if stuck open, the engine wont get hot enuf to give good heat.

Not likely it's the thermostat, as just standing there at idle, there would be no flow of air through radiator and the water would warm up.

several possibilities:
1) low coolant as stated before
2) blender door stuck (under dash)
3)heater core clogged/gunked up something fierce
4) thought there might be a switch under the hood that opens coolant flow to heater when heater turned on. Switch/lever.

An air pocket might have settled in the heater core. Run the engine and bleed out the cooling system of all air then refill it.