Well I finally managed to remove the rear parcel shelf to get at the rear speakers (8 ohm). Two fasteners were hard to locate and prise up with a screwdriver. Anyway diagnosed the problem with the speakers (one of which was completely locked solid) to be that of sand and grit finding it's way into the space between the voice coil and the central plastic support for the tweeter. After much thought the only way to fix the problem (apart from buying new speakers with the attendant installation hassles) is to remove the cone and clean out all the grit from where is has lodged at the base of the magnet assembly. There are some sites I found using Google which offer reconing services such as http://www.simplyspeakers.com/ as an example.
Will keep you informed as to my progress.

Phil

'98 S70 NA 20V 128 Klm