Installing Shut Off Switch for Courtesy lights

[1997z71]

I'm working on a 92 G20 Conversion Van. I want to install a switch to shut off the courtesy lights,so when the doors are open the lights can be shut off. I located the 20a fuse that is for the lights,also is for the horn and radio.
I cannot see the wires going into the back of the fuse block,the fuse block is hard to gain access to and hard to see. I can see several wires under the hood on the firewall.

Does anyone know the color of the hot wire for the courtesy lights? I know it's hot all the time, I thought i would wire a fused switch into it. Or if anyone knows of an easier way to install a switch,let me know. Thanks