I finally located the RABS control module by removing the access cover on the dash and using a mirror and flashlite to look back inside the dash. It's on the far left side of the access hole and is back in several inches and up almost to the top of the dash when accessed this way. This looks like the easiest way to access the plug connector for diagnostic purposes. I had already tried to get at it by removing the stereo, but I didn't go so far as to remove the mounting chassis from the dash. I assume this is how you would access it for removal.
I retrieved the diagnostic code by removing the passenger side kick panel (right in front of the door sill on the passenger side) and locating the plug connector with a red/black wire connected to a orange/black wire. When you unplug it, you use the orange/black wire to retrieve the code. Simply turn the ignition on and ground the orange/black wire for at least a second (don't ground the red/black wire - it'll blow the fuse). Your ABS light on the dash will then blink the code. Mine was three short and one long flash (code 4). The Haynes ABS Manual (all but useless) says this is a "Closed RWAL valve switch". Leave the plug disconnected while you turn off the ignition to clear a transient code.
After going through the Mitchell diagnostic procedure for code 4, which basically has you do a continuity and short check for the wires going to the RABS control valve (located under the master cylinder), my next step is to replace the RABS control valve (NAPA part number 570087 - $169.36 + $35 core). Before I shell out the bucks, does anyone have any experience with this code or problems with the RABS control valve? Since the system can only display one code at a time, could this be masking another problem?