What is the ruling on taking a sum of money from the owner of a "recovery truck" in exchange for guiding him to the site of a car accident to remove a damaged vehicle?

This transaction is permissible. It falls under the categories of brokerage (samsarah) and commission (ju'alah). Brokerage is the act of mediating between a seller and a buyer, while commission (ju'alah) is the designation of a known amount of money to someone who performs a permissible act. A condition for the contract of ju'alah is knowledge of the amount the worker is entitled to. This amount may be known by quantity (such as a specific sum for each car) or by percentage (such as a percentage of the agreed-upon fare).