Could be a lot of things setting off the Light the question is what.
worth a try > Disconnect the battery and turn on something to drain any power like the Ignition Key / parking lights to drain all power in the system. reconnect the battery and try it .
On some cars this well reset the Computer. Are some Electrical Experts that can help you here on this forum I am sure they well chime in after seeing your post . If no go then you need to discover just what is setting off the light . I seem to remember something related in the Left ( Drivers side foot-well behind the carpet Dead Pedal area ) having something to do with the central warning system but may be wrong with that.
This Link Related may help you Check it out:
http://jenniskens.livedsl.nl/Technical/Tips/Files/Porsche_928_CentralWarningSystem_Repair_rev1p0.pdf