No sound when car is running

Hi, I've just finished hooking an amp, capacitor and 12" sub in my car. Everything runs fine when the car is off, well except for after a minute or so the led goes off on the capictor, but nonetheless everything works fine with the car off. When I start the engine, the amp and the cap turn on but usually no sound from the sub. Sometimes the sub may work, and sometimes I'll just get engine noise coming through it. I've noticed that when the car is off, the amp and cap are running with 12.5V and when the engine is running the amp is around 14.5V and the cap at 15.8V. I read through the manual and didn't see anything about the amp not being able to play sound after 14V, but maybe thats the problem. If this is the problem, is there anyway to regulate the volts when the car is running? Or is there another problem?
Thanks

The light does stay on when car is running and the bass sounds just fine when the car is off.

The amp I'm using is an audiobahn A1800D, rated at
4ohm 1x600 watts
2ohm 1x1200 watts
1ohm 1x1800 watts
The capcitor is 1 Farad and the sub is dual 2ohm voice coils rate and 1500 rms.

I used 4 gauge wire for power and ground, of which the power wire goes straight to the battery fused with a 100 amp fuse. I should really use a 120 amp fuse because the amp has four 30amp fuses. The power wire goes to the capacitor and then to the amp and the ground wire goes from the amp to the capacitor and then to the frame of the car. The remote turn on is straight from the receiver as well as the rca cables.
All of the wires are run through on the same side of the vehicle. I need to switch the rca cables to the other side. The speaker wire I used is 12gauge and is wired in parallel with the dual 2ohm voice coils. So I believe is running at 1 ohm.

As for the ground, I've drilled a hole into the backseat frame of the vehicle, of which I scraped off the paint on one side of it but couldn't scrape off the other side. I then bolted it there.
Thanks