When i brought my brand new fiat 500 sport (2013) i added the interscope sound ugrade. And then found out on picking it up i had no under seat storage :eek:
So i spoke to fiat and they told me it could not be fitted. I thought it was a bit strange so asked about prices for seat base. now hope your sitting down because the price i was given was about £850.00 and £91.00 for the plastic under seat storage box.
So i then started looking for seats on Ebay. After asking loads of sellers i found one with the under seat storage fitted.
I then asked if they would take some photos of the seat base to make sure i was getting the corrtect part. The images came where helpful.
Next i asked if the seller would strip the seat down as i did not need the back or air bag or trims ect. Stated all i wanted was the seat base and storage section.
The seller said yes and sent me more images once it was stripped down to just the base.
Now thease images where the most important, as i could now see how things fitted togeather.
So now i looked at my seat base that is in my car and thought things looked the same. even two very important holes.
I then asked the seller if they would now remove parts from the seat base. And again yes the seller did this. but the price would be the same :). So i said yes and got parts sent to my home.

I will now do a tutorial on how to fit the part needed to have the under seat storage.
1: Locate a seat base. This is the hard part. Try breakers or ebay.
2: Try and get only parts you need. As this saves on postage. (flexi part and storage box)
3: Once you have all the bits lets fit them.
4: you will not need to remove the seat from the car to fit the parts.
5: Remove the seat cushion pad. Look under the seat towards the front you will see two 10mm bolts. Holding it in place, remove the bolts and lift seat cushion up. follow wiring and uncip from loom. seat cushion is now free.
The red rings show bolt location onece seat cushion removed.
seat bolts.JPG
6: Now you need to unclip side trims. You need to use a bit of force and manipulation to unclip them. (you do not need to remove the full trim from seat)

This is what you need to unclip
trim clip.JPG

And this bit make sure it moves once front bit free.
second clip.JPG
7: Now you need to remove four 13mm bots. push the seat trims away to make it easy to get on the bolts.

Located here.
seat brace fitting.JPG

8: You will now be able to remove it from the seat base, cut any cable ties holdig loom in place.
9: Now remove the O/S seat hex screw.
bolt fix.JPG
10: Now we fit the part that turns the seat into under storage type.
11: The bits in red need to fit into your old seat base. See the holes in seat base. push the part into place. This is why you remoced the hex screw. the seat has a bit of give in it now.
12: Now fit the bolts. As below. and refit the plastic seat trims.
13: Now refit your old seat cushion pad. As below.
14: Fit plastic tray into place.
15: Route any wires so they will not get damaged by seat.
16: fold seat cushion into place.

Job done.

Note. check your seat base for those two very important holes. There big enough to push a big finger into. you may need a complete base of no holes in yours.
i can not be held responsible if the above tutorial dose not work for your fiat 500.
The part used was from a year 2009 fiat 500. My fiat is 2013 1st october. sport version with half leather.

If you need any extra info please ask.