what is the best way to find out what is causing an intermittent brake light on the instrument cluster?

 

Start by checking the brake fluid level.

 

Also check your parking brake. When my parking brakes needed work (cables had to be replaced), it was an absolute nightmare to get the brake light off. I'd have to stomp on the parking brake and then get out of the truck, and lift it as fast as I could while releasing it to "slam" it up all the way to make it contact with the switch. Occasionally if I'd hit a big bump, there went my brake light because the pedal moved. Then I'd have to pull over, and do the circus act again. Very annoying!!

 

Mine did that for a while and is now almost constant. Removed plastic above dash and if you jiggle the harness plug it kicks off and on. Apparently that is a common issue on these trucks. Solution is to have re-soldered or replace instrument cluster. As a cheap fix there is a guy on youtube that stuffs cardboard between the cluster and plug to create contact. I did this to shut it off for inspection but only lasts so long. I need to do a permanent fix but 500 bucks on a 200k truck just to get cruise control back has me questioning its worth.