i use the pioneer AVH-P3100DVD in my Z and i think those can be had in your price range. it's the earlier model to the P3200DVD.
check amazon/ebay. i have a rearview camera hooked up on mine.
if you want to find the ground wire for your speakers you can also use your volt/ohm meter and measure between known ground and your speaker wire. your volt meter will tell you when you got continuity with ground... then the other wire is (+).