will more than likely be one of the cooling pipes that runs round the rear of the engine. There are some running to the plenum cooling from the heater pipes and you also have water pipes to the turbos. if you get a mirror and torch you might be able to see which one is leaking.
Easiest way is to remove the plenum when you have found which pipe is leaking and do a plenum bypass (it removes the extra water pipes) replace all the smaller water pipes while you are there. If you wish to join
ZXClub - A UK 300ZX Owners Club for Z32 and Z31 • Index page there is more information for the z32