Manual Switch for Secondary Fan

I have a 1988 base convertible last year I had a temperature switch put in that was to come on at 200 deg but it came on at 170 deg.  I saw where mechanic tied in to the relay so I wired a switch to his wires and I now control primary fan manually.  I would like to do the same with secondary but I know it is controlled by ECM I do not want to screw this up. Can someone tell me how to wire a switch that would control the secondary fan and not screw up anything else?  thanks for help.