First check the battery connections at both ends of the cables - including
under the insulation (often corrosion there goes unseen).
If all is good, jump-start and check if the alternator is charging. It is
charging if the battery voltage rises about 1V (to 13.5V or so) at about
2000 rpm with engine running, headlights on, a/c etc on. If your '99 has a
voltmeter on the dash, you can use it as a rough guide, but a real voltmeter
across the battery terminals is much better.