Bob,
When you say that you have no power to the diagnostic panel, are you referring to the Aqua-Hot controller panel in the rear-most passenger side bay? If so, and none of the indicator LEDs are on, it's possible there is no power to the Aqua-Hot itself. If you haven't already done so, check the 25-amp fuse labelled Aqua-Hot located on PCB #5 in the rear run box. Use caution as that PCB is always hot, even with the battery disconnects off.
As for the Aqua-Hot multiplex switches, the amber lights for the Electric and Preheat are just indicator lights and will illuminate whenever they are turned on, even if the Aqua-Hot has no power (they are powered from the multiplex system). The amber light for Diesel is different, it is a status light for the diesel burner and is powered from the Aqua-Hot diesel controller, the Aqua-Hot must have power for this light to illuminate.
If your tech didn't already do so, look at the Intellitec modules themselves. Each channel is fused and will have a small green LED on when selected. Since your electric channel functions, have someone turn it on and off while you watch for the green LED, that should help you locate the diesel channel, check the fuse when you find it.
-Scott