I just replaced both lines on my 240D due to 'biodiesel-damage'. Easy task, you can get to them from underneath. I'm not sure if it is as easy on a W124, but check the area right above the rear differential.

__________________
'99 S420, 155k
'91 VW Vanagon GL, 150k
'85 VW Vanagon GL, 120k
'82 VW Westy, aircooled, 165k