or, if you don't mind keeping track of the age of the o2 sensor yourself, just unplug the electrical connection to the box and remove the spinning cable that attaches the box to the speedo. This box is just a mindless mileage counter - triggers the light every x miles. Some have said that it is nothing more than an unnecessary burden on the fragile speedo gear.
(this is all based on my assumption that the 84 still used the cable speedo rather than the later electronic sender in the differential, I don't know how the later setup counts miles or if it even has this light...)
Joe