have 2 glow panels both have done this the repair isnt an easy job took 1 apart and it snapped into pieces! the green lettering is on a paper like material. use coloured acetate and a vinyl cutting printer or vinyls have seen this done with good results.
the unit it is housed in is glued to seal it hence the breakages. there is a light on either end that shines down it across reflective surfaces.
they are repairable but be careful getting the boot piece off isnt an easy task either.
best advice take your time! and dont force anything. its almost 30 year old plastic!!!