Vintage car heating

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My 1928 vintage car comes fully equipped but with no interior heating, unsurprisingly. Over the years with various vintage cars this has not concerned me at all, being part of what is expected.
However, I am now hiring out for wedding use and this includes during winter, I will need some form of interior heating in the car. Looked at the 12v heaters that plug into an accessory socket but these are inadequate. Any suggestions please.

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Thanks for reply. The car has a water pump but I have no idea about how to go about fitting a heater from it. Perhaps its time to visit a workshop.

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Thanks V8 for info, just a bit too far to travel to your workshop.