The one in my 83 SD had a crack. I fixed it with Permatex gray RTV because that's what I had handy. If I had to do it over I'd try a two-part epoxy made for plastics.

The first thing I noticed was that the hose lead to the area behind the headlight. I stuck it back in the hole in the fender but the puddle and coolant loss persisted. I pulled the tank and noticed the crack.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD