So far, I’ve come across two types of E30 rear parcel shelf speakers, that I’ll designate as “plain” and “premium.”
The plain ones are just a cone-shaped unit with a frame and a black mesh cover, and most of it is below the parcel shelf.
The premium ones are mounted in a box that sticks up above the parcel shelf, and consist of a main speaker (that looks similar to the plain-style speaker) and a smaller “tweeter” speaker unit. Each of the two speakers has a removable black mesh cover. The larger of the two mesh covers has “BMW Sound System” integrated on it with white letters. Even though each side (passenger side and driver side) has two speakers (integrated into one box), there is just one pair of connectors to the car’s wiring system, for each box.
The speaker wires that connect the car’s wiring to the speaker have two spade connectors, each of which has a different size so that they can’t be connected the wrong way around.
The passenger side speaker wiring on the car is blue, and the driver side speaker wiring on the car is yellow. Each pair of wires is twisted, presumably to counteract the effect in which parallel current-bearing wires can generate secondary electrical effects in each other.
I have plugged premium speakers into a car that came with plain speakers, and they worked fine. So, the car’s wiring seems to accommodate both types without any change needed.
Cars equipped with the premium speakers have a different style of rear parcel shelf, because the premium speaker require larger holes.