Hello from OKC. First off, There is not one simple answer.I assume that the front heat is working, if it is, then allow motor to run for about 15 minutes. Open hood,on the pass side look for the heater hoses.at the front of the motor. they will be black, one will be 3/4'' dia. one will be 5/8'' dia. both will go towards far pass side under hood,to the heater core. after motor is hot, touch hoses from motor to core, should be very hot.then look for tee fittings in hoses that allow two hoses to go to the rear of the van. feel these hoses also.if everything is flowing correctly they should be hot all the way to the rear heater core.if they are cold from the teefittings, then one or both of the tees are clogged-up.if only one hose is hot, then u have a clogged-up rear heater core.For the heater cores to work correctly, the coolant MUST be in excellant condition. If coolant is rusty looking,possible leaking head gaskets. hope this helps. be very very very careful when working with hot coolant. AC.