This is on my Prostar but the Meritor ABS system on my truck is pretty common in the industry...
For two years I've had an intermittent ABS warning for a shorted "left aux axle" sensor circuit. On the Prostar at least the Aux axle is the rear axle. Unplugging that sensor triggered the same code with the FMI for an open so the code is definately left AUX axle. Isolating the failure symptoms over time showed it only occured under conditions with a fair amount of water coming off the cab.
Driving on wet roads, splashing through "puddles" that resemble small lakes, (one delivery in Idaho I was wondering if they shouldn't have chartered a boat!) etc, no problem at all. Moderate rain or heavier or snow melting off the cab would invariably end up triggering it. I traced that circuit front to back multiple times with no joy. Cleaned and greased every connection. Replaced the sensor when I did the wheel bearing last year.
So I'm dealing with a fuel lift pump issue last weekend with the air filter and left inner wheel well removed. I noticed a wide water stain stripe on the firewall running down to the left front cab mount. Directly under the mount sits the left STEER ABS sensor connection. Clear signs of water intrusion into the connection. Clean, grease, relocate to another spot. Two thunderstorms in 3 days and no ABS warning anywhere to be seen.
Oh yes Mr Engineer, your wonderbox computer systems make it MUCH easier to isolate failures and make repairs.... I have 14 years military background troubleshooting electronics, install stand alone Engine management systems as a hobby.. and this system that fooled me is supposed to make working on trucks easier for a 19y/o tech school grad? Right...