I just got the car pretty much finished, but I notice that with just the head unit plugged into the stock harness, it doesn't get very loud and only my rear speaker works. What would it take to get it to work properly in my '94? I've done the dash conversion and have the amp...is there a harness I can buy that would go between my amp and HU?

I have monsoon b-pillar speakers, door speakers and the factory '94 amp and box installed. Dont know where to hook up the b-pillar speakers to, and my door speakers are inop. They are all pretty much brand new. I want to hook up the amp but I dont know where to get the wiring.