Well, In theory, if you bridged the 2 positive speaker leads across a third speaker, instead of directly to each other, that speakers resistance would still allow a degree of stereo to the other 2 speakers. But I've never tried it and I would be concerned about possible damage to the radio.

I don't think there's clearance behind the kick panels for decent speakers anyway.
Is mounting a pair in the under dash cover panels not an option? What about one in the dash and one underneath? If you don't want to cut holes, How about a pair of those small cube speakers like they sell at Radio Shack?

Hope this helps,
Happy Motoring,
Mark

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DrDKW