These are some speakers I found that match the classic 900 convertible extremely well
  Easy to fit - the cover is on poppers and simply prises off
      4 screws hold the speaker in - remove and disconnect the old speaker
      place the new speaker in and screw the screws back in - place the cover back on and a quick shove with the base of your hand to stick the poppers back together
      Will fit the the rear without any additional effort
  I'll install them for you if you're local to me .. or fancy the effort of visiting for a coffee to talk classic 900's
  Beware I have got a PhD in boring for England on the subject
    They will update the sound of even the oldest stereo
    Not one complaint about my speaker choices .. (perhaps I know a bit more than I thought) 30 sets sold in total
          Peak input power : 250 watts
Continous input power (RMS) : 80watts
Sensitivity (1W/1m) : 90dB
Frequency response : 45hz - 25khz
Nominal impedance: 4Ohm
Woofer magnet 10.97 oz
Directional rotational tweeter
Mounting depth 62.1mm
Speaker size 17cm (6.5')
        Fits all models Convertible (REAR only)
  1986 - 1993
  best wishes Steve
      01582 499777
  I can be contacted until 10.00pm most evenings and weekends for support or advice