If memory serves:

The Trooper sedimenter warning light is controlled by a float in the bottom of the fuel filter bowl which grounds a wire to one side of the light in the cluster when the float reaches a certain level. There are two wires on this unit, one which goes to ground and one which goes to the cluster.

Sound similar to what you're dealing with?

1988 Isuzu Trooper LS
2.8l Isuzu 4JB1-TC intercooled turbo diesel
www.dieseltrooper.blogspot.com