1989 GTU.....

On the left (drivers) side of the engine,
at the bottom there is a leak. It is coolant leaking
out of here....What is this fitting?

 

It is the hose that goes into the heater core, the other end is under the oilfilter. It is a bitch to change, Auto part store do not make the hose,you either have to get it from mazda or make one out of two hoses with a 90 deg angle. I did the second one it was not that hard but it is a tight space

 

I bought mine from NAPA... same as the one that busted.... Its not very hard to change... or at least mine wasnt

 

You should get in there and try and find the exact place where its leaking from, however from what these guys are saying it seems to be rather common.