carpc, 99 audi a3, sub not working with 4ch amp

Hi, I got a cheap 4 channel amp - the only other things I know about it is that its 'bvc450' and 400w (dont know if its peak or RMS) - it was only around £10, second hand. Basically what I'm trying to do is use all the stock speakers in my car with it. There are two in the front, two in the rear, and one small stock sub (i think only 6"). The pictures below show how Ive hooked it up, basically two channels run the four speakers (which works OK except the fronts are louder than back), and the remaining for the sub (which doesnt work). I'm using the onboard sound (High Definition Audio subsystem using the Realtek* ALC883 audio codec) from the PC which has 6 outs on the back panel. According to the Realtek software I've run a 3mm jack-to RCA from the orange port to the amp for the sub, and another from the green port to the amp for the rest of the speakers. The test from the realtek software plays a sound really softly from the sub, even if I turn up the gains, and with music its not audible. I just wanted to clarify if this is the correct setup, and if it will be possible to do this with just one amp or will I have to add another? Hope this makes sense. Thanks

its a 1999 audi a3, the stock speakers are not bose