I would be very surprised if this was spontaneous combustion. I would be very suspicious of an electrical short. I suppose all the wiring will need to be replaced anyway, so the point might be moot.

Here in the United States there are a couple of companies who specialize in cleaning up after a fire. They have special detergents and even treated cloth rags that remove all of the smoke smell. You might contact an insurance company in our area and ask them who they use for this kind of work. The company can either do a clean up for you or they can tell you what products to use for this purpose. Ordinary detergent and cleaners will not remove the smoke smell. The products they use will remove it from all the non-porous surfaces. You might still have problems with the headliner and, depending on how smoky the interior was, with the foam upholstery.

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Tim--'72 ES, '90 240