Theres loads of space under the steering wheel... Thats where I put my HU.
U will need to disassemble the standard HU unit and copy/xt3nd the wires for the power and xt3nd the speaker wires to the compartment under the steering wheel.
U can buy ready made ISO extensions and use them(with a bit of soldering).
U basically just make a copy of the power wires (1 set goes into the standard HU and the new set goes to the new HU) and just use the standard speaker wires, which you dont need for the standard radio anymore as you're going to use your new HU instead.
I can upload some pictures if you want to see it.
Heres the backside of the standard radio/computer unit, you can see the standard ISO plugs for power and speakers: