Rear Radiator Clogged, How To Unclog?

I have a 2007 Sienna XLE. The heat runs in the front seat but in the rear, it blows out only cold/cool air. After troubleshooting it over the phone with a mechanic friend, it seems that the rear radiator (maybe "radiator" is not the correct term but part that the radiator fluid flows into in the rear behind the passenger side back panel) is clogged. He said that the only way to unclog it was to either remove it completely or hook up the hose pipe to it and flush it out (which could make a mess). I called the Auto Parts Store and the person I spoke to said that it could be flushed from the engine. So, I am looking for advice on what options might exist to flush out the clog. (Trying to find the method that will avoid radiator fluid flowing all over the back seat). Also, I checked the servo motor in the back and that seems to be working okay. My mechanic friend thinks it is clogged because of the two pipes running into it the top one gets warm but the bottom one stays cool.