You have the right cable harness # for the "retrofit kit": 4F0 051 592A.

BUT, my install instructions say in turn that consists of two things, the cable and the installation instructions book. The harness # is shown in the instructions as a separate number: 4F0 055 307B. If you care, the book number is WSK 014 720A. Those two together make up the "retrofit kit" with its part number. Thus, maybe the cable alone is fewer $$, which was Harry SK's question about alternatives a few days back.

....

Current #1: '06 D3 A8L W12 brilliant black w/ amaretto & 4 seat setup, GmbH custom, CPO; modestly VAG COM lowered, H&R 20mm rear DRA's with 275/35 on OEM polished + forged 20's; S8 sway bars (see: rear bar and gen'l D3 bar info and front bar); tweaked 385mm front factory brake mod and matching rear brake mod; matching C6 A6 back headrests (better rear view: headrest tweak); owner installed AMI retrofit with part number details here; other VAG COM tweaking to suit
Current #2. C5 '00 A6 4.2 ming blue w/ beige, new off showroom floor; Lemiwinks + VAG COM tweaks; smoked tails; RS 6 OEM 18's; RNS-D (CD) Navi
Prior: '96 C4 A6 2.8Q, still w/ family; '85 C3 5000S 5 sp FWD, '73 C1 100LS

helpful cross reference to a C6 post with MMI and other TSB's that also cover D3 A8's.