ther is watr leaking at rear of enginebetween the head& the block.is ithead gskt

Do a compression test at a minimum, leak down test would be good if you can get the gauge set from a local parts store loan a tool program. If you have low compression in only the one head then yes it is probably only the rear head. However your gasket set comes with head gaskets for both heads and you are already tearing the engine down that far, you might as well do both while you are there. When you say the back of the engine are you meaning between the firewall and the engine? Or where the transmission mounts to the engine ( drivers side )? If it is at this point it is very possible just your intake gasket is leaking, this is a common place for it to leak. And many times will run down along the block where the head meets it and then down the back or front of the engine. I always advise that if you do remove the head have it pressure checked for cracks and refaced. Better safe then sorry

From what I understand most 1999 Malibu if it behind the steering pump on the block leaks a little. I was told you can let it go and it may last like that for years with no problem or get it fixed for around 7 or 800.00 I choose to leave it alone a year ago and it hasn't gotten any worse it's just a little leak so I just check the cool fill every 3 or 4 months it needs a little or not.