this should be your setup, it is how GM wired the 4th gens with 3 relays. grounding the 335 and 473 circuits should trigger the relays. you said you had constant power, how about the switched power on the trigger side? what is your CTS reading? maybe its bad and the ecm is not flipping the fans on?
you know you really ought to fix the problem and not band-aid it with a manual switch. your car is too nice for that