Below is a guide on how to wire in a set of rear parcel shelf speakers into the rear door speaker wiring. It took about an hour from start to finish including fitting a pair of Pioneer 17cm speakers into the existing holes under the rear speaker grills.
Electrical wire colours should not hopefully differ between 166 models, but for clarification mine is a late 1999 2.5.
Remove the Black plastic trim at the base of the B pillar. This is at the bottom of the internal pillar that the top of the front seat belt emerges through. You may have to ease the first section of the door cill where it meets this section in order no to break the clips.
Working from the rear of the car interiour look at the pair of electrical looms that can be seen at the base of the pillar as they exit down into the main loom. One is a Yellow wire, which should not be touched as it is connected to the front seat Air Bag sensor, further up the pillar. The second loom is sheathed in a cotton braid, and is about the thickness of youíre fore finger. This is the loom that we are interested in. Carefully peel back sufficient part of the cotton braid to expose about two inches of the loom. You are looking to locate the following wires, depending on what side of the car you are working on.
For the passenger side you are looking for a Grey and Grey/Black wire
For the driverís side, you are looking for a Green and Green/Black wire
Carefully expose these two wires as they may be buried in amongst the other wires of the loom, and connect up youíre rear shelf speaker cables with whatever method you decide. On both sides thease wires were twisted together within this part of the loom. I soldered mine, as I feel that this is the best electrical contact method, and its probably too tight to add in a couple of Scotch blocks, but you can of course try.
Was it worth itÖ.. In my opinion yes, even using relatively cheap Pioneer 17cm speakers the sound stage is a lot bigger, and I have found that the fader control is now biased toward the front to appreciate the additional speakers.
Good luck, but donít come after me if you botch youíre wiring this is only my advise and you should think about your own electrical skills before taking on this worthwhile task. Rember STAY WELL CLEAR OF THE YELLOW WIRE UNLESS YOU WANT TO SET OFF YOUR FRONT SEAT AIR BAGS.....