There are two DIN chassis radio installation kits for the A4 on the market.

One is made and sold by Scosche. It is part number AU2300B and works in 1996 and up A4's.

The other is sold by LLtek. The part number for 1996-97 is 07.027.001.02. The part number for 1998 is 07.027.002.02.

Both of these kits allow the replacement of the factory Audi radio with a DIN Chassis unit.