Check for a corroded bulk head connector which carries many wires through the fire wall under the dash. Early Workhorse 2002-2004 had the problem until they redesigned the entire connector. It can be accessed from the front and is a big job to get it open and clean/apply dia-electric grease to all the terminals that pass the wires through the firewall. Very aggrevating condition to say the least. God Luck... :)
The connector was corroding from being exposed to weather on the manufacturers lot until built into a coach which then shields it. It was redesigned in later chassis to make it more water proof.
