This panda of mine is strange, for some reason it doesn't have a boot light, another cost saving measure from Fiat I guess. Luckily most Pandas come with everything installed for the boot light except the light itself.
The photo below shows the whole in the left hand side of the boot under the parcel shelf (2). You will have the mounting but no light fitting. The two cross head screws (1) and a hole for black plastic clip (3).
(Cam C902)
Firstly you need to gain access to the wiring, begin by removing the two cross head screws (1) in the picture. After the screws are out pull the top of the carpet away from behind the plastic shelf fittings and from the rubber seal around the door frame.
You can then feed the plug, the black plastic clip and the wire thorough the prepared crosshair cut in the carpet. The picture below shows the plug, wire and clip correctly inserted through the carpet.
Make sure the black plastic clip can reach its hole in the shelf fitting (marked as (3) in the first picture), then slide the carpet back into place behind the plastic fittings and fix into place with the screws.
Pass the wire through the light mounting hole (2) and then connect the light which should, in theory illuminate. Remember that the boot light times out after a certain length of time, so don't panic! Closing and re-opening the boot should activate the light.
The light fitting slides up into the mounting and will then snap in to place. The black plastic clip that supports the wire pushes, from behind the bracket, into the hole (3).
The last job to be done is to put the carpet back behind the rubber boot seal.
The door switch to turn the light on and off should already installed. BUT...do check!
Do make sure your car has the appropriate boot latch incorporating the microswitch for detecting open/closed condition or you will be disappointed. I have read that some models don't have the appropriate boot latch.
I don't know how you tell, but thinking out loud here...I would expect the 'door open' dashboard light to be an indicator that you have the appropriate boot latch. For example, with the engine on, all four doors closed and the boot open making a red 'door open' dashboard light appear, I assume this means I have the appropriate boot latch incorporating microswitch for detecting open/closed condition. That's the test I did before buying the part.
Good luck