My 2008 Passat (FSI 2.0T) has gone intermittent on the heat for no reason. Blower motor works just fine. Temp on full heat and I get cold air coming through. Then for no reason, I'm driving along and the heat works again and then off again (mostly off). I have a big trip through the mountains of Montana tomorrow and need to get my heat working again. I do not have the climate control version, just the manual climatic version. I know these can clog up in the heater cores -- but before I go through that whole mess... is there anything else than can fail on these? An actuator? A valve? the heater knob itself? Thanks.