I have a VZ SV6 with a Sony head unit and recently purchased steering wheel control harness also.
I bought a ISO harness end (that plugs into the original cars harness) that i then wired together to the harness wiring that came with the sony head unit.
This way i effectively created a whole new harness without cutting or touching original wiring.
You can actually get these already made up from Aerpro. Check out
Aerpro Eg.
VZ ISO harness to Sony 12-Pin
Also, in regards to the Steering Wheel Harness, this is also a Aerpro product,
CHVYVZA
The problem i found with this harness is, 1 wire, the blue wire which keeps constant power to the unit for memory is missing, so i had spare harnesses around and pulled the pins out for it. This may not be required for certain Units though.
These are available to be ordered from JB HiFi, for around $120-$150 all up for all the head unit harnesses & steering wheel controls harness too.