Sub Very Quiet??

I have just installed a Fusion slim sub with a built in amp.The sub has power (green power led is on) but it is very very quiet and i can only just hear it. I have checked the power to the amp and its reading 12v on the multimeter. The RCA leads are new and are not loose. It is earthed to the floor properly ( I have scraped the paint away). I do not know what is wrong. If anyone can help it would be much appreciated.

Set your multimeter to resistance (ohms) and check the speaker inputs of your amp. If your sub is a dual voice coil (dual 4 ohm for example) it could be possible that you wired it up wrong.