psycholist

There is no rear pad wear indicator I'm aware of. The only sensor for pad wear is in one of the front brake pads. You'll see a wire going to the brake pads on one of the front calipers. It's possible the ABS pump is doing something fancy to track the position of the rear pistons, but I've never heard anyone mention this.

Give the surfaces a good clean before resticking the number plate and it should be fine - much better than drilling through all the rust protection to put in screws or rivets anyway. The sticky pads listed on this link are definitely up to the job (I've used them on a trailer), but other foam pads are likely to work just as well: http://www.eireplates.com/accessoryproducts.asp