the relay VII in the fuse box is for the 4x4 sapphire cosworth only, the standard sierra pre 89 used it but had a different arangement.
the standard sapphire 1990 on has all its abs relays on the (M) relay module which is located under the dash at the r/h side of the passenger footwell around the lower corner of the glove box. it can be lowered down by undoing a screw/bolt for easier access.
relay m12 is the main abs relay, m13 in the abs pump relay 1
and m14 is abs pump relay2,
fuses 30,31,34 supply the 3 relays with main power, the relays are activated via the earthing through the abs computer.
with your problem the fact that the light is going out and then comming back on when you drive would indicate a speed sencing failure to me and that means sensors or sensor.
the do fail completly or can be intermittant.
fuse or relay failure will generally have the light on permanent and or the break pedal very hard.
you could check for continuity between pins 22,23,24,25 at the abs computer and their respective sensors, if thats ok then i would replace the front two sensors regardless of ohms readings, if that fails then replace the back two,it is expensive but it may be the only way to sort it as the system is generally reliable except for the sensors.
let me know how you get on