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Originally Posted by padriver
E&C Data Line connects the A/C components (HVAC programmer, AC main controls, AC passenger controls) and the radio in a daisy chained manner. On cars without dual AC, there's only the connection from the radio to the DLC. It's obviously the bus that the components of the dual AC system use to communicate. Not sure what the connection to the radio is used for though....
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Not that I think it will be helpful to anyone, but as far as I have figured out, the connection to the radio dims the digital display of the electronic climate control system. It uses (as I recall) a fixed frequency, variable duty cycle signal that causes the brightness to go from fully bright (if you disconnect it) to completely blank (if you connect it to 12v constant).
This only becomes important when you replace the factory radio. The factory radio is what dims the display when the headlights come on. (I don't know why they did this.) So if you have an aftermarket radio, you have to build a module to dim the display or else you have a really bright light on your dash at night.