99 Maxima Heater Problems

[Robert99SE]

I have a 99 Maxima with no warm air coming from the heater. Fans are blowing, A/C works, new engine thermostat in the last year, new radiator in the last year, fluid flush in the last year, no fluid leaking and no fluid smell from heater vents. Does this sound like a heater core problem, a heater thermostat problem or other? Where is the heater core, and how hard is it to get at?