Clarion DB235 not dimming with headlights

I had a Clarion DB235 CD HU installed in my 91 Volvo 740 a few days ago.  Everything is fine, however, the headunit does not dim when the headlights are turned on.  Every other HU before this one (mostly Sony, in different cars) had this feature where the brightness would decrease when the headlights were turned on so it's not blindingly bright.  Is not having this feature something specific with Clarion or this unit in particular?  Could it be the installer who missed a wire somewhere?  Thanks in advance!

If the unit does not have an illumination wire in the harness, usually an orange or orange / white wire, then the cd player will not dim with the headlights. There are many cd players that do not have this capability. If you still have the instruction manual it may show in the wiring diagram if this wire is present or not. Top Secret, I can tell you but then my wife will kill me.