No cabin heat

I have an 07 SE. I've had it for 3 years. Just recently the cabin heat stopped working. All the fans work fine and the engine thermostat remains stable.

Sometimes, although not anymore, I could turn the heat knob to cool then hot, and it would get hot. Sometimes I would hit a bump and it would get warm for a second.

This leads me to believe a loose connection somewhere. Looking at HMAservice, I'd have to tear out the seats and the whole dash to troubleshoot this. Is there a specific area I should check? Could a door that routes the heat be stuck? Or is there a specific connection that easier to get to?

Any advice would be very helpful.

Thanks.