Since the turn signals blink the brake lights on the rear, the signal switch (multi-function switch) is supposed to disconnect the brake light on that side to allow the flasher to blink the light. If that disconnect is not happening it can back feed the brake lights into the front & indicator lights. I think if that happens, the center brake light might blink when you use the left signal without the brakes on. You can check the switch with a meter. The procedure is in the FSM, one linked below if you don’t have one. For ’99 it is on page 5 of the turn signal chapter.
See online read only FSMs here:
Jeep Knowledge Base
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Pick your model & year then on the drop down menu pick the chapter you need.