Hey guys,
After a bit of research on Miata.net I found the answer:
I have had the same problem with my 1997 Eunos Roadster. The looseness in the frankenstein bolt is there because the nut into which the frankenstein bolt threads has come loose from the car frame. This nut is welded onto the frame of the car, just underneath the chrome plate that the frankenstein bolts thread onto. Since the inside of the rear package tray (where the loose nut is located)is inaccessible, thus making any effort to weld the nut back onto the frame impossible, you will have to try and loosen this nut from the frankenstein bolt in order to fix the problem. The nut is accessible from a relatively small hole in the package tray trim (the metal trunk roof panel painted to the colour of your car). Get help from a friend, who will have to hold the frankenstein bolt firm with a large diameter phillips screwdriver, while you have a go at loosening the bolt using a no.14 or 15 (cannot remember exactly which one I used) socket wrench through the hole mentioned above. Holding the frankenstein bolt firm is very important as loosening the nut is quite a process since the frankenstein bolt threads were covered in blue thread loc from the factory. Tell your friend to warn you if he feels the screwdriver slipping, so as to prevent slippage and damage to the rear trunk panel. Be patient and work at it slowly, some WD-40 can be of use when loosening the bolt. Once the bolt has been removed, thorougly clean the threads of the frankenstein bolt, apply some thread loc , and after threading the frankenstein bolt back through the hole on the chrome panel, add a 1mm thick steel spring washer the assembly and screw the nut back onto the frankenstein bolt. Adding the spring washer to the assembly prevents the assembly from loosening in the future. I have found that adding the springwasher is somewhat a peculiar procedure, especially when adding it to a bolt facing downwards, and accessible only from a small crevice in the trunk of the car. But this problem can be solved by covering one side of the spring washer with thread loc and carefully placing it onto the nut which has already been inserted into the socket wrench. The adherance of the springwasher to the nut should help you thread it together with the nut onto the frankenstein bolt without dropping the springwasher in the package tray crevice.
What a pain in the ass
