I think he should check for codes using the outlined procedure where... you short one side of the inline connector to ground with key on, I believe. the light will flash out a code. in my case, it was 16(all clear) which was annoying because the light wouldnt come on all the time and it would go out on next key. it turned out to be my brake light switch was intermittent not working at key on but it would work when i go to step, 90%. the connector was yanked tight and almost pulled out of the body of it

 

well I miss spoke,
Code 16 is always present, unless a system concern has been detected and a diagnostic trouble code has been stored. This code should be used by the technician to verify that the RABS II system is OK.

Code 16 will be output by the anti-lock electronic control unit when one of two conditions exist:

1. The technician ground Circuit 571 (BK/O).

1. The RABS II Keep Alive Memory (KAM) fuse has blown.If the KAM fuse has blown, the REAR ABS warning light will begin flashing Code 16 (or a diagnostic trouble code if a concern has been detected) if the stoplight switch is depressed (F-Series), or the running lamps are turned on (Econoline).
   
   Therefore, check the KAM fuse when diagnosing complaints of REAR ABS light flashing.

The system should NOT flash a code 16 if your truck has a code stored. Apparently only one code will flash, and you should skip the first one incase the code starts in the middle (I guess?) do you need help finding the connector body? I found mine by the crotch area of the passenger under the dash, its clipped to the IP and is visible if looking down there and mine said RABS on it. one side of that connector, since its inline im pretty sure, gets grounded with an alligator clip (i found best) to some metal then the IP will start the flashing with key on

 

Glad you found the issue.