Two things to check:
1 - Heater control valve
2 - Blend door
1 - After motor has warmed up, open the hood, follow the two coolant lines from the pump up toward the firewall. If there is a valve body in the lines between the pump and the firewall you have a Heater Control Valve (for a 98 you should have one). Put your hands on the two coolant lines by the firewall and see if they are warm. If not, that probably means your HCV is stuck in the closed position.
If both coolant lines are still cold - Check and make sure the cable that opens/closes the HCV has not come off (older HCVs had cables that actuated them). I'm guessing 98's are probably vacuum controlled. The valve body will have a small vacuum line run to it. Check to make sure the vacuum line is still connected and not damaged. If it is still connected, you might have to disconnect the HCV and bypass it to get heated coolant running back into your heater core if funds are not available for parts. I had to replace mine, cost me about $20 at the parts store.
2 - If you have heated coolant running to your heater core but still no heat:
Remove your glove box and put your hand on the blend box (large black ABS plastic body behind glove box that all the air hoses run back to).
If its hot, then your heater core is not clogged (that happens sometimes, especially if you used a "Leak Stopper" in your radiator or have not changed your coolant in several years).
If its hot, that also means your blend door is not functioning. Check to make sure the door itself is not stuck in the closed position. Should be a vacuum valve body somewhere around the top or bottom connected to a lever (blend door lever). If the lever is stuck - get it unstuck and you'll have fixed your problem. If the door is not stuck, then that means you have a problem with the vacuum lines or the temperature control on the dash.
Hope that helps. Wish I'd seen this post sooner....