Start off by having a look at the wiring that goes onto the brake components under the bonnet for damage - it's easy to melt wiring if the car has been running without a turbo heat shield. Also, unplug the connectors and pull their rubber boots back to look for corrosion or terminals that have been unseated.

The yellow wire feeds 12V from the ignition switch to the pump motor relay terminal 86 (purple relay behind the glove box). A brown/black wire runs from relay terminal 85 to the pressure switch under the bonnet (5 pin connector). When the pressure switch reads low system pressure, it connects the brown/black wire to earth (the brown wire in the 5 pin connector), energising the pump motor relay. If you have a multimeter, you should have continuity between the ignition switch yellow wire and the brown/black in the pressure switch connector. If you unplug the pressure switch connector, then the yellow wire should not connect to earth.

Also, to test the ABS warning light, there should be continuity between the ignition switch black/yellow wire and ABS ECU terminal 27 (remove the ECU connector, the screw in the end of the connector, and slide off the cover to reveal the terminal numbers). If there is continuity, also check that you've got 12V at terminal 30 of the ABS ECU relay (green 5 pin next to the purple pump motor relay).

Let us know how you get on.