Are you simply wanting to replace the head unit in the dash and utilize the rest of the factory system or do you want to simply start from scratch?

The shitbox (I mean er lovely Eldorado) belongs to my boyfriend. His head unit is all sorts of [censored] up, so I just wanted to put my old sony in its place, leave the rest intact. Any way possible? ( I still maintain, I'd like to see it undergo a horrible flaming death)