glovebox light?
(forget if there is one or not...)
Footwell lighting?
Get a multimeter on it, and then switch on ignition. If it becomes live (12v) then you know it's something which comes on with things like the cigar lighter/stereo etc.
If it doesn't become live when ignition on, switch the engine on and keep monitoring. Then switch random things on and off and close/open doors until you find it becomes live. This may help you confirm what it is.
If it becomes live when the radio is switched on, then you'll know it's for an amp.