I've got the Checklist but unfortunately can't find the light!
Checklist:
Speedbrakes Extended.
Condition: The speedbrakes extended light indicates:
*In flight, the speed brake is beyond the armed position with either the trailing edge flaps extended more than flaps 10 or rad alt less then 800'
*On ground the speed brake lever in down detent and the ground spoilers are not stowed.
Speed Brake Lever..........Armed/down detent
If light illuminated on the ground:
Do not take-off.