Why is the sound for the built in dvd player not working?

I bought my truck used. The owners before me put a after market head unit in it. I am guessing that has somthing to do with DVD player not playing sound. I have pulled the head unit out. I found two sets of plugs that were not hooked to anything. One set was for the subwoofer, the other set was for the "line in". So i took a old aux cord, plugged the red and white to the line in on the back of the head unit to the aux hook ups on the dvd player. I managed to get a little sound but there was alot of static. So, how do i get a good connection for a nice clean sound?