look in junk yards or try one of the z parts stores on line would be my best guess mine is cracked too but only surface cracks if you find one please post here where you found it and how much hope this helps. http://www.zbarn.com/ http://www.datsunrestore.com/
one of these places might have it.
I have got many parts from the Zbarn.com Z junkyard. Power seat motor delivered for $25, headlight motor delivered for $25, misc interior pieces for (rear trim panel for $15 in my color delivered). Just tell Roger that you want the center piece between the tailights from an 85 turbo and he will remove it and mail it to you I bet for $25.