Some of the water pumps M/B has been giving out has an additional brass fitting that you need to pop into the back of the water pump like a freeze plug.
Theese are universal pumps that some need the plug pounded in and some have another water hose that bolts to that portion of the pump.

You may have a big hole in the rear of your water pump that you can't see but you have to remove the water pump and get that brass fitting.
They call for an orange goop to put around the fitting before you tap it in.

If you have this pump and you need the fitting, the dealerships carry them.

I have to admit, I got burned on that one myself once, and it will never happen again.
They just went to this update not too long ago with this universal pump and the parts guy did not tell me about the fitting plug so I mounted the pump in record time, feeling all good about my acheivement, but when i poured the coolant in, it started running out the back of the pump.
Say what! I said to myself, so I had to r=r the pump again and knock in that little plug.