Try using one row filtered portals to data in the Join table and the linked customer details table. Keep in mind that you can add fields from customer details in the row of such a portal when you place it on a Booking based layout.
Here's a possible portal filter to show data about the primary Driver:
renter_driver::Role = "Driver" Or renter_driver::Role = "Renter/Driver"
A one row filtered portal to renter_driver with this filter could show the name, license number, etc from Customer Deatils for the driver or renter/driver selected for that booking.
Thank you again PhilModJunk! You're a genius