need help with a diagnosis

[spewey at comcast.net (Spewey)]

The early cars just have a light that goes on when the sensor grounds. 
Within the fuse panel, this also connects to the seat belt/brake warning 
thingy in order to borrow its buzzer sound.  (Often this relay gets 
yanked out of a lot of cars for being annoying.)

| wire from sensor to fuse panel
/\
1 2

1 wire to dash light
2 wire to sb/b light-->3 wire to buzzer relay

Faults within the panel or possibly corrosion on the seat belt/brake 
light or system could be activating your light.  Check oil pressure with 
an expensive calibrated tool.