The glove boxes were _meant_ to have a light in them, but for some unexplained reason, Subaru does not install them, or even list them as an option/accessory.
However, the lamp and switch assembly harness for it is readily available, and the Subaru name and P/N for the assembly is Switch / lamp Harness PN 83081FE000.
(Your local Subaru dealer will probably not stock it.)
It comes with the lamp, switch and harness with power connector.
The hole for the lamp is already in the glove box, and the knockout for the switch is readily recognized, and both lamp and switch will pop right in and look as though they belong there, because they do.
Since the power connector that comes on the harness will have no corresponding connector in your dash, you will need to locate a hot wire to get your power source, which can easily be tapped by using a "vampire" splice.
I can provide photos etc. for the installation if you want.
I can also give you the hot wire source that I used for my Forester, which was right behind the glove box.