hi,
in answer to your questions i work for volvo as a tech and as such have come upon this problem many times. the problem is not a problem with the module as such but its exactly as you said in that solder joints on the circuit board do fatigue where the plug socket is soldered to the board and then the module detects intermittant open circuits and this makes the light come on. sometimes the module itself does fail and cant be repaired but this situation is pretty rare, 90% of the time its due to bad solder joints and when you send these away to be repaired all they are doing is resoldering the board which is not that hard to do for someone with experience with soldering.
a tech can tell if its bad solder joints because 9 times out of ten the light coming on will mean there are certain fault codes in the module (due to open circuits) these are normally right rear wheel speed sensor,other wheel speed sensors and sometimes pump motor faults.
anyway hope this answers your post, any questions just ask me..

steve.