My aux fans were not working this summer and the resistor was the problem. If the AC is on the fans should come on very quickly in stop and go traffic at outside temps above 70F or so...
You can change the resistor by removing the horn but a better way to go is to remove the headlight assembly - this greatly improves access to the resistor. As I recall the shield is held in place by an 8mm sheet metal screw.
My power input lead to the resistor just crumbled away when I touched it - looks like it had been getting hot for quite awhile - be prepared to replace the terminal lug and the ballast resistor.
BTW the aux fans made a noticeable difference in AC performance and engine temps in stop and go traffic.
57&60 190SL parts cars