Horn Button & Pressure Oil Switch
If the oil pressure light, brake failure light and horn fail to function with the ignition key in the “on” position, the electrical grounding circuit for these items should be strongly suspected. (This defect may also occur only with the side-lights or head-lights energized.)
All of the above circuits share the same grounding connection which can be found in the wiring harness in the area of the left hand front grill/ apron. Identifying wire insulation colour is black. There is a Lucas sleeve-style gang connector that shares an incorning ground wire with the head-lamp, side-lamp, and circuits from the horn button and oil pressure switch ground. Corrosion of the sleeve connections results in a high electrical resistance and inability of the current to pass to ground.
Correction is through replacement of the sleeve connector and cleaning up of the Lucas”bullet” style nipples.