This morning my glow plug light did not come on,I waited about 15 seconds,turned the key and the car started.I havw a 1989 Mercedes 190D,,,The car started and then the glow plug light came on and stayed on for 30 seconds and then went off.The car runs fine.I turned the car off and it did the same thing.The glow plug light does not light up until the car starts and then stays on for 30 seconds before it goes off.Do I have a bad glow plug,if so ,How do you test for that?