When the speed brake light comes on when flaps are lowered on winglet aircraft, we found that the autostow actuator (in the pedestal behind the lower DU, not below the floor) is faulty or not rigged.
This is applicable for NG’s with winglets prior to line number 778—– From 778 and on no autostow actuator required. Note: The auto stow actuator is also installed on all 737-700’s.
Thanks to someone from SWA.
See picture below for the location of this actuator: