Mercedes Benz repair: 99 ML 320, coolant level, ml 320

Question
hi- i have a 99 ML 320. has had 2 new water pumps in the last year and a half and a few weeks ago my coolant light came on during a 600 mi trip. i immediately looked and my coolant level was low. added H20 during the trip and watched the temp...which would rise during idle. before it would overheat i would pull over, let it cool, and add more H20 if needed. i realized at that time that the 2 front fans were not coming on. i took it to the shop(none mercedes) here in VA (the closest Mercedes dealer is 60 miles away)and they checked the fans  by running power from the battery and they worked. i also checked the fuses under the hood. they didn't want to "mess" with anything else so i took it home. that night while looking under the hood i noticed that the coolant reservoir was cracked at the line at the right hand side. i called the mercedes and told them...i paid for a new coolant reservoir and for them to install it and told them about the 2 front fans not coming on.  they said that the car was now running and that doing more checking would run into a lot more money so i should just not let it idle in traffic. i am now VERY frustrated. 1) i need to know why the fans aren't coming on 2) it doesn't make sense to me to be charged over $400 and not know what is causing the problem 3) could it be a sensor or a relay or a secondary fuse that i've read about online?
so the problem is: the temperature rises on idle and i believe if it's allowed to it would overheat and the 2 front fans are not coming on with or without the AC on. (the AC works fine but i turn it off as soon as the car is at an idle).

note: i am a woman, i am capable of doing some things, and i can follow directions IF i have them. the car has 150,500 miles, it's my only car, and i do not have a lot of money.  thank you for your help.

Answer
first off I would never take the car or suv in your case to a dealer..I known too many horror stories.. anyways back to your problem.. when your waterpump was replaced did they at least replace the thermostat..If not I would definitely replace the thermostat as far as your secondary fans you can check them by putting on your a/c if you hear a click or see the secondary fans kick on your fans are working. The a/c without the fans on will overheat the car as in everything from over 100 degrees.. MB cars idle at 80-100 degrees the thermo opens up at 87 degrees celcius.. If you have replaced 2 water pumps I would suspect it isnt a waterpump issue and if the dealer or stealer didnt replace the thermostat shame on them...The fans either or would not m,ake the car overheat..Go to your nearest oil /lube place tell them you want a coolant flush and tell them to drain the block also..(this is of course after you replace your thermostat).. Then use a scanner or your nearest autozone may help you tell them you need to know if your thermostat is opening up..They have live feed and should tell you if your thermo will open up...and of course closes..
I would aslo check the coolant is there any non color besides green in the reservoir..or if you pull off the oil cap and dipstick is there a white milky substance??Or when you drive the car is there white smoke...If all these are good and It is overheating My first issue would be to replace fan clutch..A good way to test it is to take a piece of cardboard after car is up to running temperature and put it against the fan it should without fail cut it apart if you can stop and slow it down there is a problem ..also if you have someone else turn on the car have it up to operating temp +87 degrees have them turn it off the fan should only spin 2-3 turns after car is turned off...Do the thermo first and check the exhaust pipe for white smoke and look at the coolant reservoir and oil dipstick and cap ... Also if the fan clutch is shot your idle gauge will also be erratic at stops and what not it will go from 97 to 80 degrees at a stop...If your temp gauge is bouncing around like that do the fan clutch .. Then hit me up if need directions about the secondary fans.. fix the first problem then work on the others..