My heater in my 99 dodge durango does not heat unless the rpms are up.....i think its the heater core is that hard to replace? And where is it located for sure??

This is provided you have confirmed that a plugged or partially plugged heater core is your problem, and not a vacuum problem, or heater control valve or similar is not defective. Yes it certainly sounds like some blockage/restriction there, but may I advise you to try this first, as it may save a lot of money, time, & aggravation. Go to Rona, Home depot, or similar hardware store & buy a drill pump. (about $25.00 in Canada) but there are cheaper ones around.
Remove your heater hoses at the firewall (when engine cold) and you will see the 2 copper ends of your heater core to attach drill pump to. Put one end of the pump (pressure side) on one fitting, of the core and the other hose from pump suction side, into a jug of CLR (calcium ,lime, rust, remover.) The other connection/fitting from your heater core will need a piece of hose going from it, into a pail, or another jug, same size as CLR to retrieve liquid when it flows through. Circulate the CLR through the core a few times, then let it sit in the core for a while. Then reverse the hoses at the core & circulate it through the other way a few times & let it sit in there as well. I let mine fill the core, then I plugged line where CLR would flow out of heater core & then let it sit all night in there to dissolve as much calcium etc., inside as it could. Then next day, I ran pump in both directions/fittings, again to flush everything out. Then I re-attached my heater hoses. My heat increased at least 75% at idle, because it had removed a lot of the restriction in the core.
Make sure you take all safety precautions, such as eye protection in case CLR splashes in your eyes, and make sure engine is cool, pressure is relieved via removal of rad cap & to avoid burn from hot coolant when hoses removed. My guess is that your into $20-$25 tops, and a whole lot less work. Let me know how it goes.

I'm not a mechanic but i have the same problem i took my dashboard off so i could get to the heater controls and i unplugged the fan control and the selector switch. Then i plugged them back in and the heat came back on full blast rpm unrelated so you might want to check your heater controls to plus always look for TSB(technical service bullentin) and recalls