One problem related to mid/rear seats assembly has been reported for the 1999 Audi A4. The most recently reported issues are listed below.
Driver's seat controls do not conform to car's manual. Car manual specifies a separate "lumbar" support knob. While driving I turned what I assumed was the lumbar support and found it to be the knob controlling the angle of the back seat, causing a momentary jolt, disrupting my concentration and thereby constituting a safety hazard. It turns out, there is no separate knob to control the lumbar pressure, contrary to the description in the manual.