dimmer switch

hey guys i have an 89 firebird with a 305 tbi. when i got the car the dash lights didn't work i looked at the wiring diagram and saw that they ran to the factory radio so i found the wires in the radio harness (it was brown and grey) and now the gauge lights work their bright, but the dimmer doesn't work. I tried another switch but they still don't dim is their another wire that is suppossed to be connected to those two? i don't see it on the diagram

does any one know?