1998 kenworth t800 brake lights not working

I had no brake lights on my 98 w900 and it was in the turn signal/hazards switch pigtail on the steering column. I tested for power at the air operated switches behind the dash and chased it to the turn signal pigtail/connector.

 

I'm pretty sure it's a 12v system but I'm no electrical engineer!!!

 

There service brake switch for the ECM will show 5v or less with the key switch on.
Service brake switch for the brake lights should have 12v all the time, key on or key off.

 

Use a test light to follow the power. Find the last place it powers the test light and you'll likely be staring at your problem. I've never needed to check the voltage when troubleshooting something like taillights or marker lights but I haven't seen much compared to actual mechanics. You should be able to press your brake and hear the switch click at the moment it lights up your test light. Follow the wire that powers up when brakes are applied and retest on either side of any connections. I wedge a wrench or literally anything against the brake pedal while checking along the chassis or anywhere away from the captain's chair. If you have 4v leaving the brake switch then there should be 4v at your brake lights if there's no break in the wire/switch/connectors. Just get comfortable with standing on your head and chase that wire from the brake switch.