When the air conditioner or heater is turned on, it works on all speeds but only blows out of the defrost and very little at the door vents. There is next to nothing blowing out of the front facing dashboard vents. The rear air works normal.

SOURCE: heater blows cold air 1995 plymouth voyager

With the engine cold & the radiator full remove the radiator cap and start the engine. Let the engine reach operating temperature which will open the thermostat. When it opens the water in the radiator will start circulating if the pump & thermostat are operating properly. If all is ok replace the cap and turn the temperature lever on the heater to hot If you still have no heat check the heater valve on the engine firewall. Open it manually and see if you have heat. If yes check the cable from the velve to the temperature lever.

SOURCE: 1999 plymouth grand voyager heater does not blow hot

One of two probable problems:
1. The Heater / Air Condition Control is going bad or is bad.
a. Disconnect the negative side of the battery.
b. Remove the Heater / AC face plate off the front. To do this: Remove the two (2) screws on the top lip of the plate.
c. Disconnect the electrical connectors from the rear of the Control Module.
OR
2. The Heater Core is defective and needs to be replaced.
Sorry, but it has to be noted that the dealer or repair shop ($$) can diagnose the system at the Heater / AC Control Module. Also, after you replace this Module: a diagnostic and re-calibration tests are needed to be performed.
Let me know if this helped or if you have additional questions or information. Feel free to contact me at FixYa.com!