Hi Ian...The truck itelf can tell you what the problem could be if you read the 'blink codes'.
Lift off the cover that goes over the fuses and relays and you will see a little led light next to where you plug in the diagnostic reader.
Turn on the ignition and this little red led will flash in a sequence of long and short flashes ( a bit like morse code)
If you make a note of the blink code sequence you can match it up to what it tells you in the workshop manual* and you will know where to start looking for the problem (*available on CD off E-bay)
cheers Richard
RICHARD RESCUE, September 2010