348 speedo problems

I can't find this problem on any of the speedo threads on this site, wonder if anyone has any ideas: Speedo works perfect, until you turn on the headlights/instrument lights..then dead speedo. Switch the lights off.. back to normal. I checked all those little light bulbs behind the instrument cluster, and cleaned all the connectors. No change.
Any ideas would be appreciated.

 

Figured out the problem, so if any of you ever get this yourselves, here's the answer: On the back of the speedoeter there is a kind of printed circuit board on flexable plastic. The plug from the car wiring harness connects to one end, the other goes through a slot in the speedo and plugs into a fitting inside. On it's way there it also brings power to the 3 instrument lights on the back of the speedo. I removed the plastic circuit board and made all the connections with real wire instead. Fairly easy to do as the ciruits themselves are obvious when you look at the plastic piece. Problem solved. There must have been some shorting between the various circuits in the plastic piece, although not visable just by looking at it

 

No I didn't because my multimeter died. But it was clear the second that voltage went to the instrument lights, the speedo would drop to nothing. I could run with the headlights on with the instrument rheostat turned all the way down and the speedo was fine, but turn on any juice to the instrument lights and zippo. The printed circuit on the flexable plastic sheet seems a little iffy engineering-wise, but sounds like I'm the only one who's had this problem. Hopefuly everyone else's will continue to function properly

 

Anything I can do to help the brotherhood. I wouldn't have bought a Ferrari without this site and my348.com and both have already saved me hours of diagnostic time and a bundle of cash. Now, if someone doing a major engine-out service on their 348 woud video the process and put it on Youtube, life would be perfect.