"I have a 2002 Grand Caravan- my temp guage runs about 45% when sitting idle heater blows cold air, when rpm increases so does the heat- coolant is full.
please help....

-thermostat stuck wide open-cheapest correction
-grab heater hose that feeds the heater core normally on the left if your facing the motor (goes into firewall) if hot see if the one next to it which is the return feels cool.
-that would indicate a blockage in the heater core
-if both hose are hot at normal operation temperature then you have suffient flow in and out of heater core...
-grab top radiator hose at normal operating temperature-hose should be hot and you should feel the coolant moving through radiator hose with pressure behind it..
-if there is minimal flow in upper radiator hose then water pump could be getting weak..
-If you have never changed the thermostat in the vehicle I would start with that and ensure when you change the thermostat that you fill the system entirely before running vehicle or you get air pockets in system that will be very difficult to remove and you wont get any heat at all..
-top off system and wait for thermostat to open up and then once coolant level in radiator drops down refill immediately to ensure system is full..
-most likely this will fix your problem unless you have hvac blend/recirc door issues which are all dash removal jobs..
-start off simple and eliminate maintenance that is probably overdue anyway like coolant and thermostat and you should come across your solution...

I'd put my money on the thermostat although i've seen waterpumps cause the same problem. The waterpump is pretty easy also and you get to change the coolant too!

Had the exact same problem with my 02 grand caravan...the thermostat had broken apart...

Thermostat would be the cheapest attempt to fix, but with the heat working with higher rpm sounds like the water pump is having problems pushing coolant through system. Don't forget to check your belts also.