Ford Repair: 1985 Ford Thunderbird, digital speedometer, ford thunderbird

Question
I have a problem with a recently purchased 1985 T-Bird.  It has a digital speedometer.  It works great in the daytime.  When you turn the headlights on the speedometer goes blank.  If I pull the fuse out of the tail light, it doesn't go blank.  This one has me stumped.  Any suggestions?

Answer
I had a 88 t-bird that did the same thing.  I was never bothered enough to try to figure out the problem because the car would never shift past second gear and hence did not go very fast anyway.  When I turned the lights on, the screen got real bright green but the digital readout stoped working.  However, the digital readout was never as bright as it should have been in the daytime but it was still visible.  My guess is that the backlighting is getting brighter than the digital speedometer making it seem to stop working.  If this is the case you must remove the fuse that runs the interior lighting, it would be incharge of the dash lights, the radio, heater controls, ect.. ect..  If my prognosis of the dash illumination overpowering the speedo is not the case, please email me again and I will consult my electrical diagrams.