Small water leak from the rear of cyl. head on bmw 318 i 1999 can you get to the problem without removing the cyl. head

First remove and check the bottle for cracks in the bottom and sides . Have a coolant system pressure test done and have a compression test done for head gasket and cracks. Have the radiator cap tested for correct operation

I'm not taking sides in this. So what I am going to say is more or less neutral.
There is a big possibility you damaged the head gasket when the engine overheated last week. The dealership would have no way of knowing this if it was not yet leaking into the engine when they replaced the water pump.
Once the water pump leak was fixed, the system would be running under normal pressure which could finish off the weak head gasket.
You can not see a damaged head gasket without removing the cylinder head. And you have two heads with two gaskets.
So, IF the head gasket is now blown there is a good chance they did not know it last week. It could also be a cracked cyl head or leaking intake manifold gasket. Or all of the above.

You should have a cutoff swich under dash on pass side of truck.
The fuel pump a a 7.3 1997 f250 is behind the fuel/water separator.Between the cyl heads.Remove the turbocharger then fuel/water sep.remove banjo bolt and seal ringsat rear of pump.remove line asmby
one for each cyl head at rear.and remove fuel asmby.
Ensure camshaft lobe for pump is on base circle with lobe facing away
from pump to prevent pump damg.

It could well be a PVC valve. The PVC plumbing on e46s is not one of BMW's best designs, and it needs to be cleaned out every 100k miles if not sooner.
Not likely a rear main. I have never seen a failed rear main seal on a 6 Cyl BMW.

Are you sure it is not power steering fluid? I would be surprised if a 2003 were not leaking a little power steering fluid by now if the hoses have not already been replaced.
Another common oil leak is the hose that feeds oil to the VANOS.
If the leak is toward the rear of the engine, I would look for a valve cover gasket leak.
The service engine soon light is almost certainly unrelated to the oil leak.

remove fana nd fan shroud, disconnect the upper and lower hoses, disconnect the connection from harness,remove the 13mm bolt and 3 10mm bolts,pull the t stat housing away from the cyl head,clean surface and examine for pitting (ie small cavities) on sealing surface, once cleaned install new part and fill with coolant and bleed air from system,

The thermostat and water pump are both common failure points on BMW 6-cyl engines of this era. I would recommend checking the thermostat first, then moving on to the water pump. If the vehicle has overheated several times, you may need to remove the cylinder head and have it checked for warp.

When is the last time you had your cooling system services. It could mean several things, but without more information, its nearly impossilbe to diagnose.

Could be a faulty thermostat.
Could be a leak or lack of water.
Could be a vapor lock where the system was not run with the cap off after adding anti-freeze/water mixture caused by too much air in the system.
Could be a faulty or leaking water pump.
Could be a broken fan belt or fault secondar fan.
Could be a leaking head gasket.

Hi, on these engines they have a plastic elbow at the back of the cylinder head that gat brittle and break inside the head, it may look ok but it would be a good place to start to look for a coolant leak as this is quite common. regards mark.

it's head gasket or water pump, or if you are lucky the thermostat is broken.

clean out the drain near heater box outside small rubber elbow