Head Unit Problems

Wuts up guy's, i just bought a new head unit for my 92 civic hb because my old one got stolen. I installed it my self and it worked fine until i turned off my car, when i started the car again the head unit didnt turn on. I checked all my fuses, got a new wiring harnass, and even returned the head unit for another one. It still wont work! Im taking a 6 hour trip for thanksgiving and i dont want to get caught in LA traffic without a radio! ITS TORTURE!!! If anyone knows whats going on. imput would be very appreciated, thanks.

First thing I can think of:

Use a multimeter or voltmeter to make sure that the head unit is indeed recieving the 12V constant from the battery. If you've done that then make sure the wire harness from the head unit is connected to the wire harness coming from the car correctly (not just matching colors cause sometimes they don't).