If you don't want to do it yourself, I recommend Stylyn in Bentleigh, on Nepean Highway.
I've taken my Legnum in there three times; they did my Mongoose Alarm, Head Unit and Splits. I also bonded with the guy there who owns an import (Skyline), so you won't get any 'import' run-around.
For the Head Unit install, I think it was $80 install, $20 for a sweet double-din case thingy.
Most recently, I had some crossovers installed (previous ones were faulty, just needed to swap the boxes and tune). One car audio place quoted me 3 hours labour (3 x $88!!). Stylyn did it for $80 cash - sounds awesome too!
