Normally, in order to obtain a plenary indulgence, one must pray for the intentions of the Holy Father, among other things. When the Chair of Peter is empty, how does one fulfill this requirement?
There are three tribunals in the Roman Curia that help pope fulfill his duty as universal pastor of the Church. One of these, the Apostolic Penitentiary, is responsible for matters regarding confession, the forgiveness of sins, and the granting of indulgences. For this reason is it chiefly known as a tribunal of mercy. When Pope John Paul II passed away the question was asked whether a person was prevented from obtaining an indulgence because the See of Peter was vacant. The answer was no, getting an indulgence is still possible. Here is the explanation from the Apostolic Penitentiary:
Even though the Apostolic See is vacant, the conditions of praying for the intention of the Supreme Pontiff are fulfilled (by reciting once the "Our Father" and the "Hail Mary" once; nevertheless, the opportunity is also given to the individual faithful to recite another prayer which pleases them according to the piety and devotion of each one), even if he has fulfilled the duty of his life, since the ends of the Pope's intention, the ends for which one must pray--undoubtedly the spiritual good of the whole Church -- persist.
Pope Benedict's intentions for March 2013 were published at the beginning of the year and are:
GENERAL – Respect for Nature. That respect for nature may grow with the awareness that all creation is God’s work entrusted to human responsibility.
MISSIONARY – Clergy. That bishops, priests, and deacons may be tireless messengers of the Gospel to the ends of the earth.