A few years back we had a Citroen ZX Volcane auto. My Wife loved it. However it plagued me with an ABS light on. Checked through everything to ensure no coked up connections. The Citroen was a little prone to this. In desperation I booked it into the local Peugeot garage for a spin on the 'super tell you all that is wrong' coughed up my £50 with can't find anything, must be the ABS modulator result. Off to the breakers and another £50 for a S/H unit, still the dreaded light was glowing on the dash. So I invested in a digital multi meter. The problem, a broken wire on the N/S/F ABS sensor. between the brake caliper and the first connection. A well known problem with the Citroen/Peugeot range. This was my first foray into ABS.
Often, by no means always, electrical faults are due to corrosion in the connectors. Jag owners of yesteryear would know this only too well. A wonderful car let down by dreadful lukar connectors.