Hooking up car amplifier to door speaker question?
C'mon Don. No load could "burn out those channels"? No. That's not how it works. El Duderino -- I'm not sure what you're trying to do here. If you're just replacing door speakers and you want to use the stock amp, then just connect them to the existing wire in the door. If you're trying to install a new head unit and run it through the stock amp then from what I can tell the "HU out" wires are your signal wires. My guess is they're low (rca) level inputs, but connecting to them may be a bit of an obstacle since your HU will have RCA outs. I'd try to find a vehicle specific forum and see how people have handled aftermarket HU integration with the stock amp. A lot of cars will have adapters to simplify the process.
Never give a speaker-any speaker- more power than it's rated to handle. If your head unit has 4 discrete channels and you play it with no speakers connected to any channels, those will be at risk of having no load and cause the head unit to draw excess current, burning out those channels. Front left and right channels can be controlled by the fader option when they're properly wired to speakers. Follow the installation directions for installing the new head unit, that came with this stereo. Compare that to the original schematic for you car's stock system- which you left a total mystery in this thread.