Although each party can relate the issue as they see it, they do this for the benefit of themselves and the other party. It is important for each party to understand the position of the other, even if they do not agree with it. The mediator helps the parties to move beyond the issue. He won't be making a decision as to who is right and wrong. He would approach the garage owner to see if he will agree to proceed with mediation.
Mediation depends on the willing participation of all the parties.
If one party shows no interest, the process will not work. They can hire a private mediator. The garage owner might belong to a trade association that has a list of approved mediators from which to chose or there also might be an association of mediators in your area that would provide a list of mediators. The lawyer told my dad that in our area we have to try mediation before taking the case to court. In some Canadian jurisdictions, mediation is part of the court process.