I know that there is a post for this already but...
 

heres my question on this topic a friend of mine has just bought a new car with all the bell and wistles and he would like to compete recreationally can he ues a line leavely adapter and get good to ok results ?

If you use a basic LOC with no ability to set output levels on it, you will get marginal resulsts at best. If you use a LOC like the Scosche SLC4, you will get better results again. If you use a proper interface adaptor like the Audio Control LC6i + optional bass remote you will get the best results and the abilitly to control the level of the sub.

jl audio makes a nice unit as well