I have seen this time and time again, people asking me how they can fit a sub/amp to a standard stereo – most people that have asked me seem to think there is “pre-outs” on the back of a standard manufacturers stereo, which there isnt. The only way to add a sub to a standard stereo is to add pre-outs, using something known as a “High-Level Input to Low-Level Output” converter.
I have used a number of different converters over the years, but have found the best, and cheapest option to be the Boss B-65n:
This wires into the speaker outputs coming out of the back of the stereo – I wire these into the rear speakers. There is a popular belief that you have to cut the speaker cables, and lose the function of the rear speakers when fitting this – this is FALSE, you can just tap into the rear speakers (allowing you to keep the rear speakers working, and also giving an output using RCA adapters for your amplifier).
Once this has been wired in, you will just need an ignition feed (also found on the back of the stereo on most vehicles, or from the fuse box) and you can then power an amplifier which in turn will power your sub-woofer perfectly, allowing you to have some bass while keeping the standard manufacturers stereo.
On a quadlock installation – as found in most Fords, and Vauxhalls (amongst other cars now days), this it the top pair and bottom pair wires, closest to the outside of the quadlock block (in this instance, the wire colours are Green/Green-Black, and Purple/Purple-Black):