What was it in for service for?
There is a cable connected to the back of the gauges. I'd start there and make sure it was hooked up. You'll need to dis-assemble the dash to get at this cable.
You might try and trace it out of the dash and see where the other end connects, if you can identify the cable. I'm not sure where it goes from the gauges, as I haven't ever traced it down to see where the other end goes.