The fan will not run at high speed because of a faulty rain/light sensor.
Now when I think of it, the actuation of the AC compressor could be done directly by the AC control and only the electric fan activation could be done thru the DME, and it makes sense in your case. If you dont have any other communication faults in the car next logical step will be to disconnect the RLS and see if the code will go away(and fan start to act normally). If nothing changes you can try to disconnect the AC control and check if the communication code for the RLD will go away. There is a big chance either AC control or RLS is shorting the K-line.