If you have access to diagnostics then check the codes to make sure that they are being thrown up by the shuttle valve switches.
If you haven't done the SVS modification, then I would suggest you do it now. If the fault is down to the connection problem inside the ABS Modulator then that should sort it. If the fault is elsewhere then the mod should have no effect.
Most of the time it's not even necessary to replace the switch plate with a new one, just check for correct operation operation of the switches with a multimeter, although some people would prefer to fit a new one seeing as they've gone to the trouble of getting the old one out (£30-£40).
Once the mod has been incorporated, then I would suggest that you clear all fault code indications and check the operation of the vehicle. If the fault no longer presents itself then you're good to go, if the fault indication comes back then you're going to have to look deeper, and probably dig deeper too.