I bought a Bissell Little Green Deluxe machine (as you can guess, it's American) - wet vacuum. It's one of the best things I've ever bought
When the $25 bottle of Little Green formula started to run out, I decided to try Simple Green instead, diluted twice as much. As a bonus it's <ahem> green
It works really well on seats etc. particularly if the spot/stain is sprayed with cleaner half an hour beforehand. Overall there is an amazing amount of dirt extracted - the water usually looks like an espresso coffee - brings up the fabric colour and eliminates odours!
The machine has become rather noisy (I switched back to the Little Green formula, but that made no difference except that it didn't work as well). Strangely though, as the seats start to dry, the remaining dirt rises to the surface - so a second pass the next day sorts that out.
With carpets, I get the best results if I remove the carpet from the car and spray with Simple Green, then clean with a waterblaster (pressure washer!) This may sound extreme but it flushes all the dirt away... with the carpet in the car, it's harder to extract the dirt - especially from the closed-pile budget carpet used in cheaper cars - longer-pile carpet seems easier to clean, paradoxically enough.
I agree that without using a wet vacuum, the foam aerosols are the way to go.
Cheers,
-Alex