Some info HERE
From another post...
"You likely have a vacuum leak. Trace the vacuum line from the cruise control in the engine bay and look for broken/disconnected lines and connections. Start at the cruise control module on the passenger side of the engine bay, and work back. It's vacuum line will go to a T-connection along side the fender, that t-connection goes forward to the vacuum storage box for the cruise under the battery tray, and back around the coolant overflow tank to another t-connection next to the firewall just to the passenger side of the engine. Trace all the lines and connections that t-off the line that feeds the cruise as well until you get to the intake manifold. Also check all the vacuum nipples on the intake manifold to make sure they aren't leaking. A couple are not used from the factory and have rubber caps on them that become brittle and break over time. Check all the hard lines, t-connectors and vacuum nipples for cracks and check the soft lines as well. Check the vacuum storage box under the battery and make sure it isn't cracked/broken either.
My cruise has stopped working a few times, and it has always been because a vacuum connection has come disconnected somewhere. Usually the first t-connection next to the fender directly off the cruise module, or the t-connection on the firewall on the passenger side of the engine. Sometimes it is because I dis-lodged them working in the engine bay and sometimes they have just come loose on their own. "