Is the period during which I abstained from listening to music considered to have been in vain merely by listening to it in a colleague's car, and what should be done to avoid such temptations?

Listening to music is forbidden (haram (definition)), and your listening to it in the car does not negate your previous avoidance of it. However, you must repent from this and increase good deeds, as Allah the Almighty says: "But whoever repents after his wrongdoing and makes amends, Allah will accept his repentance. Indeed, Allah is Forgiving and Merciful."