I have repaired numerous breaks on my 1979 300D. First you have to locate exactly where the breaks are so you can dab on the liquid copper defroster stuff. Turn on the defroster. Using a voltmeter and the + probe, gently slide along each element. Good elements will show a gradual voltage drop as you move from the + side towards the - side. If you find a sudden drop to 0 volts then you found a break. You may have several breaks on a given element so you may have to repeat the procedure until you find them all. You have to be carefull with the liquid copper stuff and don't turn the defroster on until it has dried. Otherwise your repairs will go "poof" and you will have to redo your repair.