Nah you don't need to worry about disconnecting the battery, that will just reset all your computers and annoy you

This is an awesome site for wiring diagrams, The Install Doctor http://www.installdr.com/

Click quick radio wiring info and its got info for various models of cars
Here is VW
http://www.installdr.com/Harnesses/VW-Wiring.pdf


Mk4 is a pain in the ass for trying to fit a head unit in, especially Alpine with their bulky heatsink out the back, it's tight back there!