Car Audio Q's

Hi, don't know if anyone will be able to help me out, but i have some car audio issues. (Also, it's my first time posting, please go EZ on me )

So i have a Monoblock in my car to run my subwoofer. Currently when everything is hooked up, i get no sound out on my subwoofer, I've checked my input signal and it works just fine (I have a 4 channel as well and tested the RCA's on that). The monoblock switches on and everything, it also doesn't go into protect. My first thought was that the monoblock was blown, however, i took it out, hooked it up to a PC power supply just to see what happens, and when i did this, i got sound out.... put it back into my car, and once again, no sound.

Is there anyone that might have a clue about why i get no sound out when it's installed on my car?

Edit: I also know that my sub isn't done for as i hooked it up to the 4 channel for testing purposes and it played just fine