You can try a voltmeter, check each line for voltage.  Most likely, a
line has opened, there are easy repair kits to patch the broken lines.

1]  Turn on the rear defog

2] Ground one end of a test lead or light, [12v] and very carefully
touch the other probe to each element line.  Since ya already know what
lines are dead, there's where ya start.  HINT~~~~~~~  Wrap piece of
alum. foil around the +probe, and press the foil against the wire.  A
12v test light works better than a voltmeter for this.  

3]  Guage the brilliance of the light as you move the + probe across
each suspect wire. R to L.  Move the probe around.

4]  If the light is bright at both ends of each wire, look for loose
ground.

5] If the light dims or goes out as you trace the line R to L, you
found a break in that line, and it can be repaired with a kit at most
parts stores.

The kit consists of a "template" that overlays the broken area, and a
mix of silver plastic and hardener [or something very similar], you
just lay the template over the broken area found by the probe, and
"paint" the mix onto the break, insstant patch.

It's easier said than done, though, sigh.  Finding the breaks can be a
bitch, I used a magnifying glass with the probe to zero in where the
break or wear was.  patience was my virtue.