>   I've just started using my rear window defogger (the electric grid)
>   and I've noticed that only one "line" in the grid appears to be
>   working.
>
Yes the rear defogger is juat a resistor that is painted onto the window. If
you have only one line that appears to be working it could be true. The
painted surface is very easily scratched, which will prevent individual
lines from working. It is easily repaired. Your local auto parts store
should have some "rear window defogger" paint. Next you have to locate the
"open" part in the line. Try the good line with a test light. Attach the
ground to one side and gently move the other end of the test light along the
line. It starts off bright and gets dimmer as you go along to the other to
the other side. A bad line will be bright until you cross the "open" spot
then the light will go out. Tape both sides of the line on the glass and
paint the open. Follow the directions for drying time. HTH
Greetings from Michigan's Upper Peninsula
Jeff & Jean-Marie Diebolt
jdiebolt@up.net