Check continuity between pin 2 or 17 (check both) of of the display control PIC and pin 10 or 6 of the ribbon cable on the display. 2 goes to 10 and pin 17 goes to 6. Sorry, I just can't remember which of the two lines it is at the moment, but I'll try to look at one a bit later and tell you which it is. It's best to check these continuities all the way down to the connector on the mainboard (as you did already). If this all checks out, you need to establish that the the display control PIC is actually running. Without an osciiloscope or frequency counter, this could be tricky.

Hope this helps,
Stu