Hi, welcome to JustAnswer!. This is Ed.
The test you performed with the instrument cluster was just... for the instrument cluster. Code 999 means the test found nothing wrong.
To find the reason for your CHECK ENGINE lamp illumination, we'll need to try something different. I'm not sure if it will work in the 2K model year, but let's give it a try anyway.
Begin by rolling the ignition key from off to on three times, leaving it on (RUN) the last time. Don't crank the engine, just cycle the key. It'll go LOCK-RUN-LOCK-RUN-LOCK-RUN. Complete all three cycles within five seconds.
Watch the same odometer window in the cluster for code display, shown in "P-code" format (ex. P0123). If it doesn't work the first time, try different cadences (quicker, slower) or try rolling the key back only to the ACC position instead of all the way back to LOCK. Sometimes it makes a difference.
The 2000 model year was in its infancy for P-code self testing and not all models had this enabled, so I'm not sure if it'll work. If not, you'll need to get an engine scan, which many auto parts stores will do for you in the parking lot, free of charge. Be sure to write the code down exactly as shown either way and then we'll proceed.
Thanks and good luck!