Fiber optic cable on gauge cluster?

[morglan]

I swapped gauge clusters last night on my 82 K10, and the oil pressure gauge and water temp gauge read all the way to the left (high). After some research, I found that it could be due to a faulty ground. I looked around, and found a black wire with an old male end still attached coming off the top left corner of the cluster (which had apparrently broken off my old cluster, because there wasn't one there.)

Assuming this was the ground wire, I cut off the old male connector and stripped the end, and found what appears to be a fiber optic cable--what is this?

ALso, am I correct in assuming that my gauge trouble is probably due to a missing grounding? (The amp-meter works, and the water temp gauge worked on the old cluster, but the oil pressure gauge didn't.)

Thanks!