Fix your butchered loom first and leave out the sound. Solder and shrink sleeve all joints. Every where he went you follow and repair. Once you have done that then consider a replacement cluster if necessary. Follow all wires colour for colour. Once that is done then turn your attention to the audio 'installation' (I use the word loosely). Check all earths and plugs for damage - ensure they are clean and tight.
I take it the PO was either a radio installer, alarm installer or a farmer. LOL!
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i