Your year of 307 will have the multiplex wiring. If you simply plug in a new stereo to the 8 pin plugs (ISO) it will cause problems. You have two options;
1) Use an interface for the steering wheel controls, depending on what head unit you are fitting. This will sort out the multiplex problem and create an ignition supply for you, as well as obviously making the steering wheel controls work. Plug and play installation. Any of the parts ending ...006 in this link will work;
http://www.connects2.com/c2search.as...=223&modID=543
Then go to the homepage to find where to buy.
2) Rewire the power ISO. Removing the multiplex wiring and connecting an ignition switched supply from elsewhere.