Finally some time to research it, I see an expert electronics repair shop has detailed the problem as the FETs that produce the high voltage needed to power the cold cathode fluorescent bulbs that form the backlight. If one fails and the voltage drops, the backlight will stay dim. If both fail then you have no backlight.
Supposedly the circuit is very compact and tricky to repair. This tech says only someone with the right equipment and excellent skills has a good chance. Of course, these are his words.
I see a handful of used ones on eBay for about $1,000. Someone has a pair for half that, but says one of the displays is dim and he doesn't know why. Well, we do now.
My dilemma is that this happened after we started a 7 month cruise (now on day 10). I am OK with dim, but it the flybridge display fails completely, I will have only the inside helm station, which is not quite as safe for close maneuvering.
BTW, I agree with Terry that our 10-year old units are generally not worth repairing.
m/v Diva Di
Punta Gorda, FL
2006 PDQ MV 34 - hull 91, 75HP, 3-blade