There's no room to keep the stock HU. Where would you put it in there if you replace it with a double DIN NAV unit? Thus I removed all non-essential stock parts, which left the hvac controller and the tuner board which supplies power to the hvac controller. Everything is modular in that it is connected by ribbon cables. I was able to remove stock HU, cd drive, satellite unit w/o issue. The hardest part was keeping the tuner board connected to the hvac controls. The ribbon cables that connect it are very short and finding a longer set of ribbons was near impossible. In the end I just reinforced the stock ribbons and jury rigged the tuner board to the hvac controller, duct tape to the rescue more or less. I also had to stuff the steering wheel control module in there, further taking up room.
I haven't had one issue with hvac operations since making this thread so it's been working. I have to open it up sometime as the steering wheel buttons are intermittent. I knew I should have soldered the connections instead of crimping.
Correct me if I'm wrong, but MY08-14 have a tuner board that is separate from the HU, no?
On the '06 & '07, there is no external tuner board; rather, everything HVAC either passes through (or is processed by) the HU's main board.
I haven't confirmed 100%, but my understanding through recent trial-and-error is such... you have three systems working together, the Auto AC Unit (ACU) located by the blower unit (below the glove box on LH-drive models), the audio head unit (HU) and the Control Panel (CP). From an off position, when you press the AUTO button on the control panel, the CP tells the ACU to turn itself on and go into auto mode, and the ACU sends two signals out in response: one to the CP instructing it to light the LED on the AUTO button, and the other to the HU containing the temperature settings and current fan setting data (which is indicated on the temp/fan dials). The material difference between the two signals is that the former completely passes through the HU while the latter is actually processed by the HU (and thus, the HU then relays a separate signal to the CP instructing the CP what to display on the temp/fan dials).
Similarly, when you adjust any of the dials (temp or fan up/down), the CP again signals the ACU directly with the adjustment signal (via pass-through), but in response this time, the ACU only sends the new display data to the HU (which, again, then sends the signal to tell the CP what to display on the dials) because there are no LEDs to light.
So if you simply cut power to the HU on the 06-07, the HVAC will still work and respond to the controls because the CP and ACU communicate directly (via pass-through the HU) on everything
except the temp/fan displays; however, because the temp/fan displays are controlled by the CPU on the HU, the temp and fan settings in the middle of the dials would go blank if main power to the HU was cut (or in my recent experience, a failed HU).
Accordingly, if you remove the HU from the '06 or '07, you're going to lose all HVAC functionality and display completely... you could easily regain functionality by bridging the pass-through signals, but getting the temp/fan displays to work without the HU seems like an epic-level hack.