Dimming aftermarket head unit on a 2005?

Forgive me if this has been asked or addressed somewhere before, but does anybody here know of a documented or undocumented wire we can tap for 12v as a dimming line in an '05 Prius?

My understanding is that the dimmer signal for the stock and JBL radios comes from the CANBUS, so isn't available to use for an aftermarket radio.

Obviously, I cannot tap into that for this purpose, but I was wondering if there was a line, like from the dimmer wheel, I could tap as an equivalent signal.