I had the same trouble a while back (97 dsdp), the solar panel active light never seemed to come on anymore.
I climbed up on the roof for a different reason, and noticed that the solar panel was very very dirty and had collected water. Cleaned it off and the panel worked again.
I think the solar panel design might need a little refinement (don't know if it has gotten refined in later years)
The panel is flat, with raised edges. It collects water that doesn't have any way to drain off, and the water also attracts dirt and dust. If the water evaporates, the panel is dirty and doesn't let light through as easily.
So clean the solar panel thoroughly and you might have better luck.