I think that light indicates over boost from the turbo. I know the earlier models have a red light in the same position to indicate over boost (>= 14.5 psi) but also a green one to the left to indicate when boost is active. Some good information in this thread about an hdj81 but the cluster pictured is not identical to yours Turbo dash light question HDJ81
That light illuminates when it senses the turbo pressure has met or exceeded about 14.5psi. These trucks come from factory pushing about 11psi.
Do you have a boost gauge? Has there been a manual boost controller fitted to the truck? If the answer to both is no, I'd pull the intake tube from the airbox to the turbo and check the condition of the vacuum hose that connects the turbo cold side to the wastegate actuator, it's either blown off one end of got a hole in it is my guess.
It lights up when turbo is boosting...if you have a boost gauge, you can check how much boost your turbo is doing...or loosing boost if something is wrong with the turbo or hoses. Agree with Ian on checks as well. Just to add, check if there is any leaking oil on the turbo too