Do you have steering wheel controls?

If yes, check your on /off button.......while driving over 25mph, press and hold the on/off button...now look at your cluster.....is the "cruise on" indicator lit in the cluster? If yes, release the button...does it go out? If yes, you need a new on/off button(have replaced many of these).....

You can verify this......drive over 25 mph.....press and hold on/off button and light comes on.....while traveling over 25 mph, and holding the on/off button down, with other hand press and release the set button.....if cruise engages, now release the on/off button......if cruise disengages, this confirms the on/off button is bad......

Very easy to replace.....

If you don't have Steering wheel controls or the upper test does not work, then you can check your fuses, check that your TCC/Brake switch is correctly adjusted, and also check for PCM codes that indicate the cruise has been disabled....

1998 LeSabre has CC on the all-in-one turn-signal lever along with wipers, washer, high-beams, and the kitchen sink.

Does cruise control work at all?

Does the light come on?

Have you checked fuses?

You can disconnect the harness from the cruise module,
and check the voltages from the fuses,
cruise lever switches, ground,
and brake/tcc inputs...