this is the type of thing you need. not sure wat country your in so i cant honestly say where is your area.
its normally known as a high level to rca converter. self powered if possible or will be a pain to install.
thats the one i have. although try find one with a variable volume control so you can get the nest sound out of it.
if your still struck get back to me and Make sure the unit is placed behind the stereo and attach to the wires behind the head unit not near the speakers.
GOOD LUCK! it will attach to the speakers normally setup as the back speakers so you can turn the sub down using the fade of the rear end. and then the rca leads plug into the unit which go to the SUB!
IF poss use a power cap which will stop unwanted battery DEATH!