I'm troubleshooting heater issues on a 2003 Honda Accord EX v6 with dual climate control, where heat comes out the passenger side, but does not come out the driver side. Primarily, at this point, I would like help being directed to clear instructions on how to take the dashboard apart to get at the heater unit to look for clogs or non-functional doors that do not seem to be diagrammed anywhere -- especially this mythical "blend door" that is always mentioned. Where is this mythical "blend door" and how might it possibly be stuck, if everything external works?

Progress so far:
Originally there was effectively no heat out either side.
Air flows, and changes per adjusting the fan settings.
I flushed the heater core, and that made heat start coming out of the passenger side.
I replaced the heater control valve (in the engine bay), just because I had all the hoses disconnected from the heater core. The original probably worked. The new better be working now it's in place. (I could watch the internal ball move while I had the piece in hand.)
I cleaned off the air filter above the blower.
I am fairly certain that all motors work. I have taken off and checked both the driver- and passenger-side temperature control motors, watched them work, moved the levers manually (incl the lever to the heater control valve). All heater door levers (that the motors move) seem to move solidly, and make thunking sounds when I move them. I can watch the recirculation motor work and see the flap moving back and forth. I watched the passenger-side motor move incrementally while adjusting the passenger heat in dual mode. I moved the lever it controls back and forth and observed the passenger side changing from AC to heat and vice versa. I can see the mode motor working, and testing the mode, I can feel air change from top to bottom in @ 4 phases. I haven't taken off the mode motor to be 100% certain everything works. But I can't see the point unless moving one of the levers manually might help me feel something wrong.
(Note: The radio display panel doesn't work. So I can't see any indicator lights or directly tell what mode things are in.)

AC and heat come out passenger-side; only AC or ambient air come out driver-side.

I've removed everything on the dashboard that I can, including the vent covers (driver and passenger-side), everything in the middle of the console, from the gear shift on up to and including the vents covers + top section covering the middle air shafts (I plug the display back in to test motor controls); removed the passenger airbag unit; and everything under the dash to get at motors, driver- and passenger-side. Can anyone please point me to good instructions for bringing the rest of the dashboard forward?

I have the "Heater Unit" diagram. That is not helpful except to confirm all the external motors involved. Is there anything that diagrams inside the heater unit, or is that always assumed to be "one piece"?

Please and thank you!

 

You might try Helm Inc.com (Helminc.com)and see if you can get either the paper version or the CD version of their Factory Service Manual (FSM) for the V6 version of your car (might be called out as a supplement). I know for the previous version (6th gen or 98-02) that Helm offers both a 4 cylinder version, and a V6 supplement that is very thick (over 1300 pages long), and covers the entire car. That's the best I can do. You could even try looking for it on e-bay.

 

Excellent. That may be the reference I've been looking for. Thank you.