I recently reinstalled my 1979 speedometer after a North Hollywood odometer repair. I als initially did some cross wiring and nothing worked right. Then I went back to the Bentley schematics and got it right. Fortunately I did not burn anything. My two illumination light bulbs were then hooked up to two black and blue wires. They could be tested for 12V with the light switch turned on. If 12V don't arrive at the speedometer illumination bulbs something went wrong down stream. Of course the right brown ground wire has to be connected to the speedometer case. Incidentally later than 79 models also have another wire connected that has something to do with a mileage counter.
Here is my 79 SC speedometer hook-up:
