I have a bathroom with a light, gfi plug and switch they used a three wire wire to run from the light to switch the white and black run through the gfi plug the red is maretted together in the gfi box and keeps going to the switch. In the switch box I have two whites and two blacks and a red, the other white and black come from another circuit how do I connect them ?

What you posted is not quite logical. The two wire cable at the switch could be bring power in or it could be taking power out. Sounds more likely it is taking power out to some other location. The only way to tell is to describe the wires in the light and how they are connected.

Right now everything is diconnected as I replaced the light and switch box to add a bathroom fan and new light. In the light there's the white black and red from the switch box and the white and black from the fan and white and black from power.
With all the whites twisted together and all the blacks twisted together in the switch and light the fan works and so does the circuit the white and black in the switch box goes to but when I connect the blacks and red to the switch and light the light doesn't work.
The fan has a separate switch.