Yeah, you need to look on the tv to see if it has audio out plugs (these are usually red and white rca victor female plugs. Then on your stereo/dvd player (whatever is providing the audio to the trailer speakers) there should be counterpart audio in female plugs. You have to cable them together. . . . or if possibly they are hooked up, you may have to designate what speakers to use on your tv (usually something like internal, external or internal/external). If you have a Jensen or Touch Audio, then you have to remove the unit to access the audio in ports. Simply use a screwdriver or butterknife and carefully pry the trim molding from around the unit, The screws will be exposed. Just unscrew them and pull the unit out.
Just remembered, if you have the touch audio unit and there are audio cables already plugged in to audio in plugs on the TV, move them to the audio out plugs (there are no audio out ports on the touch audio unit and so they are often installed incorrectly.