If someone has used tape and silicon sealant on the glow plug relay,I would certainly suspect it as being defective.They do go bad and I have replaced a few. The MB part number is 001 545 98 32. You can get one from one of the MB parts places in Atlanta,Ga.They cost about $100.
Current to the glow plugs stays on for quite a while after the light in the dash goes out. So don't be in a big hurry to hit the starter as soon as the light goes out. After the glow plug light goes out and it is quite,you will hear the relay click off.
Good luck To you.
Bill Lewallen Lexington,Ky.