I have a Sennheiser HD 202, which has served me well for the last 3 years.
Recently, I tripped on the cable and broke the left ear wire (or made it loose at any rate). This led to intermittent song playing in the left ear, which is extremely irritating!
Here is what you need to know to fix this.
Then see this link on instructables on how to solder wires.
Only catch being that the HD202 wires have lacquer on them. You will need to burn off the lacquer with a lighter or a match. To double check, after burning the lacquer check continuity with an ohm meter!
Also, since these wires are exposed, it is a good idea to heat shrink them.