Originally posted by Dieselrover:
On the OM603 engine, as the 4 and 5 cylinder Mercedes, you want to tap into the heater hose where it emerges from the head - so between the head and where the heaterhose enters the firewall/bulkhead. It's easier on your 6 cylinder OM603 engine since that hose start behind the oil filter housing - on the 5 cylinder OM617 engine you need to take care exaxtly where you place your T or you'll hate yourself every time you (try to) change the filter.
So, you install a 5/8 X 5/8 X 3/8 T into that hose, and that's your coolant supply. For your return T, you need to install it *after* the little 12V circulation pump and before the waterpump. If you install it before the 12V pump, you'll only get circulation when the heater is on. Usually this hose is 5/8" ID also, but sometimes it's 3/4" - so check.