The usb wires are connected to the usb test points on the motherboard.
Here is a video with the usb port in action:
On the back of the phone i installed a solar panel.
However the efficency of this solar panel is very low, so it would take a realy long time to charge the battery, but it's good to be there just in case. In this case i soldered two connectors from the + and - of the usb test points that connects to the pads of the solar panel which i sticked on the back case. Nokia dsigned the charging circuit in this way: in order to charge, the data pins of the usb must be "shorted", so i soldered a switch that is doing that, when i need to charge with the solar panel (you can see me using that switch with the stylus in the video). The battery is also soldered with wires to the pads, so i can reach the switch without turning the phone off. In this way i can also hot-swap the sim card (i tested and it is working). Here is the video with the solar panel:
The last modification is a hot-swap micro sd card slot soldered on the pads of the old one.
i did this becase i found very unconfortable to always remove the back cover in order to insert or remove the micro sd card. So i removed the card slot from an old lg phone and simply soldered it on pads of the original one. I cut a little space on the back cover, and now i can swap the micro sd card a lot faster and easyer.
You can see more of my mods of n900 here: http://talk.maemo.org/showthread.php?p=1352112
In spite of all being one of the greatest phones, nokia n900 is getting older, the technology is moving on realy fast with phones that have quad core cpu's and up to 3gb of ram. However there is still hope, the guys of maemo comunity are working on a new n900: the neo900. Here is the website: http://neo900.org/
I, however, decided to move on and get a samsung galaxy note 4. Now will i be able to modify it both hardware and software like i did with the nokia n900? Stay tuned for the answer!