How difficult is it to remove dash completely? And will I be able to access the firewall in the upper right hand corner when facing the steering wheel?
Just like VQPOWER said, it is a real PITA. There are a lot of pieces that have to be removed in the proper order. Since there's a lot of plastic, it's very easy to break some of the snap-on connectors. Because of the many steps for removal, you need to download an FSM for your car; it has a detailed description for dash removal. Download an FSM from here:
The late model Altima has a major wiring harness going through the firewall on the driver's side just behind the wheel-well inner fender protector. Just loosen the protector and pull it forward and you should see where the electrical harness is. This should work out real nice for you as the battery is also on the same side.
A note of caution when taking things apart - some of the wire clips (e.g. the zip-tie clips on a standoff that snap into an oval hole, such as on the cable going into the steering column) are not sold as replacement parts unless you buy a new wiring harness. So don't cut them off with a wire cutter. Carefully take them apart so that they can be reused again.
While it is defiantly not the easies project but it's not as difficult as it seems from the posts above. If you are a bit handy and feel comfortable with basic tools you shouln't have issues with taking the dash off.
Me and a friend (who knows nothing about cars, he was there for "extra pair of hands" which you'll need for this project) took it off and installed it back it in just a few hours.
Wait, you're just trying to move 0g wire?? You really don't need to take dash of for THAT! There's a LOT of poeple who have aftermarket sound systems and none of them removed the dash. Am I missing someting here??
If you get it for close to free - sure.
I don't think it makes sense to actually buy it with all the work involved and the fact that you will still have white pieces inside your car. Dash alone doesn’t completely convert your interior into fully black.
EDIT: there's no dashes on ebay right now but last I checked they were crazy expensive. Mostly due to airbags deploying in most accidents so when you actually find one out of salved car that didn't have those bags deployed it'll cost you.