Need Instructions to replace stereo of 89 gp.

I want to replace the radio and cassette player of my 89 GP but I don't know how, My brother is helping me but there is no current going to the stereo and cassette player but there is in the fuse inside the glovebox so we thought that we could get curent from the ash tray and use the car speakers but we don't know wich cable is current and what cables use from the radio and cassette player anyone knows the instructions how to do it, The previous owner removed the ash tray and installed his stereo there but I think he got power from the ash tray cables and had speakers somewere in the car because there are no cut cables anywhere and can't ask him because he moved out of the coutry.