donnaj, to reset the service engine you need to remove the side kick panel on the passenger side. you'll see emission maintenance thingie which is the red box type thing. lift upward and remove the box.. there's going to be a landrover sticker on it.. with a paperclip spread out, either poke through the sticker and reset or take the sticker off and reset with the pin/ paperclip.
Least that how it is on the D1's, not too sure for the D1 but I suspect it *should* be the same...
Also to clarify, as it seems like there's alot of UK member/ drivers here when I said passenger side I mean the RH
That light on your 99 Disco is a check engine light indicating a fault has been store regarding a problem with the engine. You can't turn it off, you must go have it scanned, find out what code is stored, get it fixed and then have it turned off.
Have you noticed your truck running poorily since the light came on?