I'll be retrofitting rear pads indicators next time I'll change brake pads.
I always thought that A8s should have brake pads wear indicators FOR ALL 4 WHEELS, not just the front ones -- these are QUATTRO cars after all !
The idea is that armoured A8 D2s came with rear brake pads with wear indicators. Because the same rear callipers were used in armoured and non-armoured vehicles (discs of different diameter but the same width of the discs and the same callipers), the brake pads will have the same geometry as in normal A8 cars, but will come with wear indicators.
Physics: The heavier the car, the larger diameter discs were installed, so that braking pad grabs the disc with bigger leverage, farther from the disc centre, hence better braking efficiency at the cost of faster wear of the pads of course. This is why armoured cars came with wear indicators.
There is retrofit electrical harness in corrugated conduit part number 4D4 971 261 that is designed to fit without having to solder anything. Plug and play into existing wear indicator harness in engine compartment. Connection point D55 in ELSA. There is existing male <--> female electrical connector at point D55. All you need to do is to disconnect that connector and connect harness 'into' the opening, so it'll look like legacy male <--> new female ------ new male <--> legacy female arrangement. Harness then goes to rear axle area under the car and connects to rear brake pads. Electrically all 4 sensors will be daisy chained and connected to the same "bulb" on instrument cluster.
Costs: TPS quotes -- brake pads 4D0 698 451 E: £206.33 (available, but a bit eye watering), electrical harness 4D4 971 261: £53.20 (backorder from Germany).
Pads are still available from eBay sellers (old dealer stock, etc).
http://www.ebay.de/itm/Audi-S8-A8-4D...item3f2ad6852e
http://www.ebay.de/itm/Audi-A8-4D-Br...item3cd43a1d70
http://www.ebay.de/itm/Satz-Bremsbel...item35c3eaef63
Because no one needs those brake pads they sit on shelves for years. I just purchased 4 (!) genuine sets from eBay for £50-80 each from UK and German sellers after some haggling -- (before publishing this post of course)
Sellers think no one needs those pads, so they frequently appear at low cost. Just wait and grab them -- all good things come to those who wait.
If there are people here who are seriously thinking of adding this feature let me know and I'll do a write up when I finish.
Connection D55:
Retrofit harness in a box:
Male and female connection to D55 at the bottom of the picture,
two connections to rear brake pads in the background:
