I stole this from the net, hope it helps.
Check that STOP fuse (not BRAKE fuse) is good and 12v present.
Check if CHMSL (3rd brake light) comes on with pedal.
If not, brake pedal switch is bad.
Be aware brake lights go through turn switch arm, as do HAZ.
Check if HAZ lights work in rear.
If HAZ work, wires from turn arm to rear, plus grounds are good.
If HAZ does not work at rear, but 3rd brake light does work,
have helper step on pedal while you listen and feel with fingertips
on the brake light relay for click as it turns on and off.
If no click, unplug brake light relay on firewall above brake booster.
(Suburban doesn't have that) There are four wires, two are Yellow.
Both yellow wires should go hot when pedal is pushed.
If not, you have an open wire between pedal and relay.
If only one yellow wire is hot, the splice that joins them is failed.
It is buried in the loom.
If both yellow go hot with pedal, check that the black wire for relay
is making a good connection to ground by using ohmmeter.
The actual ground bolt is on the top rear of engine.
If both yellows hot and ground is good, relay is likely bad.
Use caution with this step, forcing wire into a socket can hurt it.
If you have working 3rd brake light and yellows are hot at relay,
then with relay still removed, connect a jumper from
either YELLOW to the WHITE wire. This bypasses the relay.
The white wire is the brake feed to the turn arm.
If brake lights now work as they should, relay is confirmed bad.
If brake lights still do not work, but HAZ does, re-install relay.
Go to steering column in cab and you will see a 10wire connector there.
The white wire from the relay is the top one.
Check if that wire is getting hot when pedal is pushed and relay is installed.
If not, wire is broken between realy and there (not likely)
If hot and HAZ work then most likely "multi-function switch" (turn arm)
is defective.
I hate those bloody scotch locks, I throw them over the fence, too.
