I have an 88.5 Samurai JX.  I'm in process of trying to repair the instrument illumination lights and discovered a small slider switch mounted under the drivers side dash just to the right of the steering collum with three small red, white and blue wires coming from the instrument cluster.  Anyone know what it does?

It resets the check engine light.

Oh! That would probably explain why the check engine light is now on!

The light is set to come on at certin mileage points. I haven't done this in a long time as my Sami has been brain dead for almost 10 years but you need to slide the switch up and then back down to turn the check engine light off. Can't remember if the engine needs to be running or just the ignition switch on the run position.

Just slide the switch and leave it.  If you slide it and then slide it back, I guarantee the check engine light will stay on.  The engine and key can be in the off position for this procedure.

If the light is still on, after you slide it to the opposite position that it is now, then that means it was not tripped by a mileage interval and that there is something wrong somewhere and you need to start troubleshooting to isolate the problem.