It is relatively simple. Open the trunk or hatch. disconnect the connectors from the bulbs. then take the 4 nuts off of the studs that hold the taillight assembly to the rear panel. with the nuts removed you can just pull the outer taillight away from the car.
nothing more than that. You do not have to remove the rear bumper as you do on the later models. if there are any interior panels that cover the rear taillights then they have to come off but I don't think that is the case on the 99.
hope that helps