Where do I attach C-Wire in this old Rheem Air Handler
This is easier than it looks even

Simply nut the blue wire in the four-wire cable to the junction of the white wire from the two-wire cable and the brown? wire going into the air handler.  We know it's your C wire because that two-wire cable has only one place it can head: to your outdoor unit and its contactor.  Since the red wire in that cable is the Y wire that switches the contactor, the white wire, and by extension the wire connected to it, must be C wires otherwise the contactor would never work.