I just looked it up in the parts catalogue, the Cabrio did get the bigger spline from 88 and up. Still, it looks like this is a sleeve like mentioned.

26 mm sleeve up to VIN 15-K-022 258, p/n 191 419 514 A
45 mm sleeve from VIN 15-K-022 259, p/n 191 419 514

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If you have a 5 cylinder, you don't need a stereo.

1999 Audi A6 quattro
1990 Volkswagen Golf CL Syncro
1987 Audi Coupe quattro