I completely missed these posts somehow, sorry guys
The Isuzu doesn't have a service light, so presumably it never needs servicing
The timing belt light is driven from the odometer and counts 100,000km's then comes on. To disable it you remove the cluster, remove a small screw from one hole on the PCB and relocate it to a second hole. That resets the counter, and when it passes the next 100,000km's you just swap the screw back to the original port and it resets the count again.
However, most people don't know that so they just pull the bulb

Some ball bag had removed the bulb from mine...