Audio system info

Does anyone have any knowledge about the wiring for the sound system, in particular the rear sub speaker. For a 1992 Toyota Soarer UZZ31 or Lexus SC400?

I have a Limited model with the airbag suspension, it came out without the optioned factory sub speaker. Instead it was fitted with a air purafication unit under the rear shelf. It does have the hole and the factory speaker cover for the sub, just no speaker. The main head unit has the EMV option, but the TV does not work. However the surround sound does work although there is no adjustment for sub speaker level.

I have since pulled out the air purafier and installed a 10" sub.
What I need to know is, will the factory wiring harness and plug have been installed even though the sub was not in the final options? If it is, would there be an amp installed also?
If the answer is sadly "No" to both these questions, does anyone know where I can take a signal feed from, so I can add an amp for the new sub? Can I take the feed from both rear speakers and use a bridgable amp so as to get both channels lower frequencies into my sub?

Help on this would be appreciated.

sorry this doesn't really concern your questions, but since it's the subject line i was going to use...i'll just ask my question on here. I just bought a 95 SC300 and was going to put an aftermarket head unit (cd/mp3 player) in the dash. Every place i've gone has told me there's no harness kit that fits the SC300. Does anyone on here know of any other solutions, other than having one custom fabricated? Also, are the speakers powered by amps, and if so, will adding a head unit be problem with matching up the wiring?

Thanks.