Have Toyota Questions? Ask a Mechanic for Answers ASAP
Are you 100% certain that there are no air pockets in the cooling system? Have you noticed any leaks recently or done anything with the water pump or radiator recently?When you say that the temperature gauge seems to be a little low where exactly does it rise up to? It should be about midpoint slightly down from midpoint on the temperature gauge.
Roll under the vehicle and you will locate 4 aluminum lines going to the back. Two of these are for the air-conditioning and two of them are for the heater core. Make sure that at least two of them are hot to touch when the engine coolant is at operating temperature. One of two things is going on either the system has air in it not allowing the coolant to circulate to the rear heater core or there's a mechanical problem with the air blend door located behind the right quarter panel in the air-conditioning box. Remove the radiator cap and run the engine at 3000 RPM and make sure all of the air is circulated out of the system. There's also a technical service bulletin AC001-06 that deals with a problem with the AC amplifier which causes the rear air to not be as warm as the front heater. You can do a Google search for this technical service it's for 2004 through 2006 van. It may or may not apply depending on how bad your heat differential is I would start with making sure there's no air pockets system first let me know if you need any additional information on this issue.