try the the wire that goes to the glow plugs there is a ring connetion on the right off the glow plugs that is under pressure when the air-box is fitted over the top off the plugs if yours is the same . Had the same problem on my transit and ford could not find it just about gave up on it ,but decided to change the glow plugs as a last resort and when we came to remove the wires to the g-plugs the ring connector broke , so we fixed that and left the plugs to see if that had been causing the prob and hey presto never had the glow plug light come on again that was over a month ago try this before you go to ford they wanted £58 every time they had to plug it up to the computer good luck