We have a Rheem Classic unit and the brown wire that comes for the house to the unit is not connect to the main unit. There is a yellow and brown wire that has nothing connect to it and they brown wire has white and red wires coming out of it, Which wire goes to which? The termostaf kicks in but the unit does not turn on and it also controls the heating unit.

Please help

How come the wires are not connected? Were they connected originally? How come the color change?