yea you need a line output converter like like mentioned, if it were me and you wanted to keep the stock look id leave it alone and not add a sub, if you still want a sub adding a after market deck would give you the best sound at the end....you can hide the deck, glove box would be perfect. then run ur rca's off that, add the amp and then the sub
something like this is easy to get to and not hard to pull off