If it's the same system that's in my E-320 it's as described above with the coil that runs through the washer reserve tank that heats it from the antifreeze out of the radiator.

The tubing that carries the washer fluid from the tank up to the squirt thingies also has a heated metal rod running through it to prevent freezing in the tube as well.

Always be careful not to crimp this tubing!!! I've been told by my mechanic that it's extremely expensive to have to replace either the tubing or the water reserve tank.

As to whether the fluid is heated in warm weather I don't know.