how can I fix my 98 plymouth voyager heating?

I have a 1994 plymouth voyager 3.3 and I have some heat.I replaced the thermostat and the control switch but no luck.and I dont think its the heater core cause I was told if it was that there would be a leak on the floor.I was wondering what else could be wrong with it and why am I not geting any heat at all? thank you for you time!!

Dennis lentz
December 2008