IIRC it was the Nissan Primeras dominating BTCC in that era wasn't it? The Audis did OK, but nothing special on a dry track.
As far as being fundamentally similar, well, the monocoque and block were the same but the rest was all custom. I recall speaking to one of the engineers at Prodrive (where they built the Mondeos) about it, and he told me that they'd get the monocoque chassis and engine block only from Ford, then they'd go along to the Ford factory at night, reset all the panel forming machinery to the absolute limits of tolerance and fabricate special body panels (to get them as thin and light as possible). All the suspension, gearing, brakes, everything was custom built.
The guys who built the Nissan cars told me that it took 800 man hours to install the roll cages in the bare chassis!
/edit No, you're right about the Audi domination, and I'm thinking of later seasons.