We only see cool times part of the year but it is my experience that the fans come on low speed (due to A/C pressure) very shortly after the A/C comes on. Usually before the engine warms up. This doesn't happen in the winter but when its 90 out the fans should spend most of the time on.
To verify the fan function go to the reciever-drier and view the two switches. One has short wire pigtails and the other doesn't. The one with the pigtails is the pressure switch. Separate the switch from the wires and put the wires together (electrically). The fan should run on low speed. If not check the fuse the ballast resistor and the relay in that order. Actually by=passing the relay to verify thatthe fans work is a good idea unless they have been seen to work on hi speed.
Bosch Master, ASE Master, L1
33 years MB technician