What you need is a Neon Assembly Housing Lens, not a lens for the ballast. The ballast is merely the power supply that feeds the neon beam, and you'd be lucky to have/find an OEM ballast that works flawlessly today as they're all over 20 years old at this point.
SYLVANIA, the producer of the rear lights/neon assembly & front headlights, stopped making them a long time ago. and I doubt that you were able to obtain a replacement without going through FORD, let alone an individual component of the housing, when the assemblies were available.
I remember awhile back I saw few videos on YouTube about people modding their Neons into LEDs, meaning that they had to tear the thing apart.
This cat has you send in your neon so that he can convert it for you. Not sure if he still does that, but he obviously knows how to pull the assembly apart.
This next guy offers a bit of insight in the comments section into what he did to get the lens off, and like you, also had a crack lens that he wanted to work around.
Though, in the end, it might be easier to find a replacement assembly online or nearby yard and spray-paint it to your color. Duplicolor has some decent matching FORD colors, though your best bet would be
Automotive Touchup for best color matching.