No, there is probably not a relay for on/off
The Cruise Control Amplifier is what you are controlling and it is all done on one wire, plus Ground.
It is voltage based, when you press the ON button it gets a 12v "signal", as you said momentary, that wakes up the Cruise Control Amplifier, it then waits for lower voltage signals, on that one wire, SET, COAST, RESUME, +/- each have a distinct lower voltage signal the Cruise Control Amplifier can ID, the OFF button Grounds that one wire, this puts Cruise Control Amplifier back to sleep(off).
So you do need a momentary 12v on that one wire and it can't stay on or it would block reception of the other signals.
You could setup your own relay to do that but my suggestion would be to replace the buttons.
Also your 2003 Ranger might need a new Brake switch on the master, there was a recall for 2001-2003 Ford Rangers, the switches leaked and could cause a fire, call a dealer and have them check your VIN for recalls.
This is part of the Cruise Control system which is why it popped up on my search.
Last edited by RonD; 12-23-2014 at 11:17 AM.