Reach under the water pump and feel the weep hole. It's about at the 7 o'clock position looking into the pulley. If it's wet, it's the water pump. There's an o-ring between the water pump and block that might be leaking. There's also a back cover of the water pump that can leak. Are you sure it's not the heater hose that goes into the side of the thermostat housing?

I don't think there's a freeze plug anywhere near the water pump. Check out the pics at photos.yahoo.com/tabijan in the 91 300SE water pump album. Might give you some insight.

Check the archives also. Within the last couple of months someone posted about replacing a freeze plug for $70. Might be the same problem as you're having.

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