The white wire is the brake pad wear sensor, when your light comes on then the sensor also needs replacing since it gets damaged. The abs sensor is behind the rotor, but it is fine if your abs light doesnt come on. As far as saving your rotors in the future they are wear items and will need replacing from time to time. If you replace your pads when they get low(like when your light comes on) then you should be able to get a few sets of pads per rotor. Hope this helps.
1985 190E 2.3-16
1957 220S(running one day, I hope!)