DTC4-Grounded Antilock Valve Reset Switch

My truck is a 1992 c1500 (120k) with RWAL.  The brake light in the instrument cluster came on about three weeks ago.  I notice that if I set the emergency brake the light would get brighter.  After ignoring the light for two weeks, the light began to flash a code at me (one long flash, then three short flashes).  From what I can find, this error code is DTC4-Grounded Antilock Valve Reset Switch. 
My problem is that my Chiltons book does not explain where this switch is and my local Auto Zone and my local Chevy parts dealer do not know what this part is.
Does anyone know where it is, how I can test it and what I need to tell the dealer to get the right part?

Thanks,
Mike