I have a trane netral ac xl 1200 more than 15 years old. Works fine except the past week the air handler in attic keeps running continuous. It's on auto . I replaced the thermo , worked OK for a day but it started again. I was told that it could be the relay on the unit. If it is would you be able to tell me where it is,I can replace it, I'm handy. Thanks

Access the control panel in the attic unit. The relay will be contained in there. Usually the blower door or the control access door will have a wiring diagram. MARK all wires before removal or you will be calling a pro to straiten it out.