Hold on! Before you go replacing the water pump, you need to go out and check the fat bypass hose that connects the water pump with the thermostate housing. If it is damp or has dried coolant residue on the rubber, try tightening down the clamps. I had a similar situation, where there was coolant leaking on the passenger side (off the alternator bracket in fact), and it turned out that both upper and lower clamps had worked themselves very loose.

Good luck,

Greg
'84 300D