Andy:

I too had very intermittent ABS failure light for the last six months, I assumed it was gunge on the ABS front wheel commutators, which I had about 3 years back. Then it went totally and BIG SURPRISE my speedometer quit working. The sensor feeds both ABS and speedo. I never noticed the speedo failing as it was only a temporary break in the circuit It turned out both wires were broken at the differential sensor plug, but would still make occasionly. They were broken by fatigue not corroded, I replaced the connector and then put RTV at the connector wire junction to reduce the strain at the abrupt change from wire to where it goes into the connector. My first break was at 304,000 kms and almost 11 years... with my improved strain relief I figure I am good for at least the next 500,000 kms and 15 years. This is a rather minor design fault. It might be worth it for anyone to just add a bit of RTV at the junction of the wires going into the connector at the differential. This is for models with ABS for sure and may apply to some non ABS but the non ABS I checked in the wreckers did not pick up the speedo from differential.