The alternator signal to the tach is a single gray wire (like Mark said). But the Tach gauge itself has its own (+) and (-) leads on the back side that are fed from the engine panel. The gauge (+) and (-) leads don't come directly off the trailer connectors but are usually daisy-chained from the posts of other instruments on the back of panel (this daisy-chaining makes the leads prone to more than one possible loose connection). The quickest way to troubleshoot the gauge ground may be to temporarily string a new ground wire between the tach GND and the engine block. If that fixes the problem, THEN start working your way through the chain of connection behind the panel.
Tach signal wire
Gauge "Pos" (= IGN), "Gnd" and Signal posts. You can also see the Range/calibration adjustment screws Christian mentioned.
Daisy-chained wires
If re-doing the engine panel (in whole or in part) it can be helpful to collect all the grounds from the panel onto a ground bus bar. Here, the gauges, panel lights, blower, and panel accessories (temp and pressure monitors, bilge warning light) each have their own path to the ground bus. This makes future troubleshooting much easier.