On a Mk 1 Focus ,the speedo sensor sits in the gearbox. Depending on what gearbox is fitted, these can be a real pain to fit . Some are a push fit with a locking pin and some have a securing screw. However. the abs system is completely independent of the speedo circuit .There are 4 abs sensors ,one at each wheel. If the abs light is showing, then a fault code must be logged in the system.

Just an alternative thought. Mk1 dash has problems with corrosion on soldered joints. I did one some years ago, had erratically jumping dials, cannot remember about lights misbehaving. I guess lights can come on if current finds the lamp provides an easier path to earth. The fix involves removing the instrument cluster and cleaning the circuit board inside it. It is fiddly but do-able and costs nothing, the corrosion is usually easy to see and it is where current is not going where it should, or going where it shouldn't. Theres lots of guides about it on the web. If you try it take a picture of the needles before you remove them so you can replace them correctly on their spindles. You will lose trip miles when cluster is disconnected but total miles will be retained.

Good Luck