It maybe that the relay has failed ?
The easy way to check is to listen for the 'click', the relay is on the bulkhead inside the engine compartment but you can normally hear the click from inside the cab.
It's around 10 to 12 seconds after you turn the ignition on...
You can also check with a multi meter, the 12v will be turned off after 10 or 12 seconds.
New relays are around £50 so check with a mter first.
Remove and clean the contacts, examine the wires etc, relays do fail - if it's on all tye time you will kill the glow plugs after a while...
PRC6913 - Britpart, Allmakes and Land Rover options for PRC6913 Relay - Glow Plug Timer - 200Tdi - 300Tdi - Td5 | Def - Ds1 - RRC - EXPRESS Shipping - UK, EU and Worldwide.
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Glow plus are part# ETC8847 - go for Bosch or BERU not the chineseium versions