Check the heater hoses at the heater core, both should be VERY hot to the touch, if one is hot and the other is cooler, the core is restricted. If they both are HOT, then its an issue with the temp blend door in the heater case assembly.
If you have further questions on this, please feel free to reply.
Thanks for asking!
Try adjusting the temp blend cable.
Thats a tough one to test, but one thing for sure is its VERY comon for them to become restricted.
Sometimes you can try to flush the core by removing the heater hoses and rig up a garden hose and run water through it.
If it helps, then you know for sure its restricted,,,plus you will be able to see the volume of water that passes thru it.