The wait to start light is completely independant of the glow plugs. The pcm will command the glow plugs on from anwhere between 2-120 seconds. If the pcm deemes it needed the relay stays on long after the wait to start light goes out.
The PCM uses the engine oil temp sensor (EOT) to determine wait-to-start time. Jon is correct that glow plugs will remain energized after the light goes out. You may be able to find some data as far as resistance-versus-tempurature so you can ohm out your sensor to see if it is at fault, or potentially a broken or shorted wire to the sensor may be at fault. If anyone has better ideas, I'm sure they'll chime in.
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