I purchased a used sub woofer for my 92 SC400 and it was a 10 inch 1 ohm speaker. There is also a 8 inch 2 ohm speaker for some models. You'll have to take yours out and measure the diameter of the cone itself. Do not measure the outside diameter of the frame as it is a lot larger than the speaker cone itself. The coil impedence is written on the back of the spaeker, either 1 or 2 Ohms. You can't replace it with just any after market speaker because of the impedence of the subwoofer amplifier. Which is located on the rear deck next to the sub woofer.
PS, make certain you have a bad speaker first before you replace it. The amplifier may be bad instead. If you a distortion problem look to the amplifier first. I did have a bad amp as well.