That's odd. The 1800E has an unusual '2 speed' rear defroster hokked up via a relay and three wires to the rear window. One wire leads to the entire right side of the grip. The other two wires lead to the upper and lower halves of the grid. In 'high', both left hand connections are gounded, and the right hand side recieves current. In 'low' the right hand connection is open, and one left side connection receives current, the other remains grounded - resulting in 1/2 the current flow because of the double resistance due to the current making a double pass through the grid. I've never seen another car go to that trouble, and I don't really see the point. I guess the theory was to turn it on high to burn off frost and heavy mist, then on to low for extended driving?

In any case that still doesn't explain why yours would be operating in such a hit or miss pattern, only if just the upper or lower half were working. I just happen to know so much about the 1800E defroster because I spent a while testing why mine doesn't work at all. I finally determined that the contacts at the edge of the window got the proper current, but that the grid passed not a single bit of current. All the lines are there and visually unbroken, but my guess is the metal has been leached away be decades of windex window washing?