Rheem HVAC Blower works in all modes, but nothing else

[kc2z]

I have a Rheem HVAC model # rgda- 075a-cr. where can I get a repair manual for this unit? The unit blower works in all modes ( auto-heat, auto-cool, and fan only). When the unit is in auto the heating and cooling do not work,and there is a clicking noise in the four plex box inside the cabinet. The box has a transformer at the top of the box, and some kind of relay at the bottom. How do I test these components?
Please advise!!!!!
Thanks for all your help!!!!!
kc2z