Auxillary cooling fan conversion to manual switch

[hsahai]

I have 560 sel 1988. It is running hot 100+ degrees in summer and stop in go traffic with A/c on. Auxillary fan did turn on but A/C was not blowing cold air. But normal running on highway is ok with nice cooling. So all the a/c swiches, relays are ok. My question is this. Is it possible to add manual switch to Auxillary fan so that I can turn on the auxillary fan whenever I want keeping automatic functions intact. I just want to add manual mode switch. I am thinking to splicing wire to Auxillary cooling fan with a manual switch lacated near the instrument panel if possible.