Series One Sprinter Heater Plugs ???

[Watto]

Hi everyone , I own a 98 208d sprinter the only problem i have is the heater plug warning light doesn't come on when i turn the power on ,but after the engine starts it comes on then stays on for a minuite or two then goes out .
The engine runs fine and starts ok ,any ideas plz ?

[euro II 312D]

may be a faulty timer or temporizer i dont know how to call it, but in my 312d it is a grey box under the hood below the battery, that box governs the time for heating plugs, i put off the cap once and saw corrosion in it, cleaned it with wd 40 and sealed it again with gasket silicone, still running good.