90 300td battery draining please help!

I read many posts here regarding glow plug operation. I believe my car is powering the glow plugs during driving after the car has warmed up which is draining the battery to an unusable condition ie even the
radio wont work.
The battery has been checked and is less than 2 years old. Thea alternator is an uprated (90 Amp) 5 mths old.
Besides the ign key, what controls the demand for the glow plugs? (ie between the ign key and the glow relay)
The glow light does not always show before a start and shows other times.
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