Why can't I ever get aftermarket HU dimming to work?
 

In the past 20 years I've installed about a dozen head units in several different cars. Everything has worked fine every time except I've never gotten dimming with the headlights to work. I buy the wiring harnesses, match everything up, but the illumination wire, for whatever reason, has never worked. What am I doing wrong?

it could be a number of things..

in pioneer's for instance, there's a submenu that allows you to change weather the radio pays attention to the dimming wire or not. if i remember correctly, the oem setting is to ignore the dimming input..

but overall, my experience with MY2000+ pioneer and kenwood units is that the dimming feature just barely works-- the radio's that i've used have just barely changed intensity-- if they were at 100% brightness, they might dim down to 90% brightness..

so because of that, i never really used the illumination wire with those decks anymore, or would connect the wire, but disable the dimming feature within the radio..