The O.D. light will come on when you push the button. When you push the button, the light should come on, and the tranny "Should Not" go into overdrive. May be your bulb is burnt out. On a 94 I think the button is on the dash, and if I remember right, Your button should have the light in it. Later mods the button is in the end of the colum shift lever, and is called "Cruise_Haul", on the dash.
The tranny automatically resets to O.D. whem the Ig sw is turned off.
IN other words pushing the button should keep the tranny from going into O.D., and the light should be on.
Hope this helps.
barracudadave67 aka Dave C.