Cruise control not working?

Perhaps the four volts is not there with the ignition switch on if the cruise control switch is off.

As for the location of the relay, that should be listed to the right of the dashed box on a part of the diagram not shown above. Based on that dashed line, it appears it is in the same box as the fuses, ... In other words, the fuse box.

I've been overlooking the non-functioning cruise control. I use a Chrysler DRB3 scanner for my vehicles. Among other things, it lists the "reason for last cutout" for the cruise control. If you're sitting in the shop, it will be "vehicle speed too low". If you're on the highway, and it lists "ignition switch off", you know to look in the power supply circuits. I'm pretty sure GM's Tech2 scanner has the same capability. If it lists "brake pedal depressed", you would know the two symptoms share a common cause.