Good question. I recently went through a thrash with my speedo, and to make a long story short, my charging system would not work with the speedo cluster out of the vehicle. Reinstalled it today, and it works fine. I think it has to do with the relay that confirms voltage from the alternator (I think it extinguishes the batt low light on the dash shortly after start-up). After studying the schematic and troubleshooting the charging system until I was blue in the face (and replacing the alternator, ouch) I came to the conclusion that the connection to the gauge cluster must be there for the charging system to work. Good luck.

BTW, unless you have a tach, DON'T connect the red and white wire to anything when you reassemble, it's hot off the coil and will short out the chip--oh that's right, I was told you I would make a long story short...