Head Unit turns off after installation?

I bought a JVC KD-R206 head unit and installed it in my 1994 Holden Apollo (basically a Toyota Camry). But when I turn the car on the head unit turns on but after 2-3 seconds turns off. The only button that works it the display button and I can insert and eject a cd perfectly fine but nothing else works. To make sure it wasn't the head unit itself I tried a spare head unit that I had and it just turned off and nothing else worked. I've been to installdr.com and the wiring seems fine so what could be the problem?