jo's54,
That particular wire must be energized when the truck is running for the alternator to output. If you can identify a SWITCHED terminal that is "hot" only when the key is ON, your home free. Remember, if this terminal is common with the coil in anyway, you must use the diode, or the alternator will backfeed and keep the truck running, even when the key is switch off. This was your original complaint I think!
Stuart
