I actually did this job last weekend, ie to remove both sensors and clean the frame and sensor tabs.
What I did find is on th battery side there were 2 additional earth wires, one comes directly from the bat negative terminal and the other Im not quite sure but it is one of those thin braded cables. Both these are also fixed to the rail at the sensor.
Unfortunately I cant say if my problems are solved because Im waiting for my speaker amps, so the seats are out and part of the dash is still stripped.
Maybe this is the additional earth you are trying to find?
I have a 1990 so should be the same