Take off the lower trims and remove the speedo cluster. Might have to take out the radio too. Once you can see check for soldered in joints, taped up bits of loom and boxes with lots of black wires hanging free. You will soon see how the standard items differ from the aftermarket add ins. Make notes of what you take out and what wires were cut to fit that item. Decide what you still want installed and carefully repair the damage left by others. Solder and shrink sleeve all your joints. Take it steady and you will be fine. Practice soldering out of the car if you are unpracticed in the art. PM me if you wish.
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