1st step is take it up with the inspector. It is their job to find these things out, and what you pay them for. The guy who inspected mine discovered the broken audio unit, so they replaced it. However my guy didn't realise the central locking didn't work and I never chased up anyone about it, so it still doesn't work (well, intermittently anyhow) 2 years later
Yes, every 2nd hand dealer has a statutory 3 month warranty on every vehicle they sell. If they don't want to play ball, tell them you have spoken with Consumer Affairs and they advised that the dealer fix it quick smart, otherwise they will be getting a letter from the ACCC.
My advice is if you can get the $600(ish) out of them install a Nissan double din kit and put a nice touch screen DVD player/radio in like I did similar to this : http://www.ebay.com.au/itm/260817904589?hlp=false
For a small amount of out of pocket coin you will have a good setup.