Right I've bought an alpine w203ri but the power leads on the harness arnt in a block which colour goes to what in the car look

If you get the head unit model number, google it. Download the manual, that'll tell you what wires do what on the stereo side.

Last Page on below link

http://www.alpine.hr/fileadmin/user_upload/manuals//car_audio_manuals/CDE-W203Ri/OM_CDE-W203Ri_EN.pdf

But where does the yellow wire off the head unit go to and all the others lol.

The yellow should be the memory and requires a permanent connection to the battery, so start with testing the wires in the harness for one that's always live when the ignition is off and key is out.
There should be one, if not run a wire directly from the yellow to positive on the battery (+)

Buy or borrow yourself a multimeter so you can test which one is permanent live & ignition live.

Alternatively you can make a live tester by soldering 2 wires onto a 12v car bulb / led