I thought in Fra they only drive you by car from fct not from fcl.
FCL B lounge they will drive you, if you are not departing from a B gate. (Most international flights do not depart from B gates)
You can always very simply check beforehand which gate you are leaving from, and if it isnt B you can save yourself a 10-15 min walk, and the possibility of getting lost by just going to the B lounge, which is EXACTLY THE SAME as he FCT!!!