Introduction
Jimny does hot have a rear cabin lamp (neither in the trunk nor above rear seats).
Most Jimnys have a factory-made hole in the body above the rear left window, which was meant for a rear cabin lamp. However, only some "Japanese domestic market" Jimnys have that lamp installed. It is probably possible to order it from Suzuki dealers.
(See this Link for details and part numbers.)
That lamp has only two switch positions written on its body (ON and OFF), and no "DOOR" position. However, it has been confirmed that the intermediate "DOOR" position actually exists in the lamp's switch, but is simply not written on the body. Therefore, this lamp can work in automatic mode to light only when the doors are open.
Solutions
If you can not obtain this OEM lamp from Suzuki (or if it is too expensive), the best solution would be to obtain a cabin lamp taken from some other vehicle model, and install it in the pre-drilled factory hole above rear left window.
The rear cabin lamp from Suzuki Vitara Mk2 (a.k.a. Grand Vitara 1998-2005) looks like it would be a good fit. Bosanek has acquired one and is about to try to install it.