I just noticed in your post that you mentioned an “illumination +”.
The positive is constant (from pin 4/phone connector) and the ground is switched/dimmed in the backlight circuit. Just use a good switched 12V+ on pin 4 (phone connector) and dim ground on 3.
BTW, a good source for dim ground is the Gray/Blue/Brown wire on pin 3 of the original dash defrost switch.