The CAT ECM must be programmed to "Latch Mode" for jake operation when cruise control switch is in the "ON" position.

A: This is why the cruise doesn't work = thinks brake pedal is constantly being pushed down (depressed)

B: This is why the jakes are being constantly activated, too = thinks brake pedal is being pushed down (depressed) {When cruise SWITCH is ON}

He seems to be on the right track, in thinking it is a "brake" issue, that cause both problems.

I'm guessing that you have an electronic engine, for the year 2000. There is a good chance, for its brake pedal ECM signal, it might just be using a simple air pressure switch, set at 3-5 psi that triggers an "Closed" circuit  "Open Circuit"   (with brake pedal depressed) = that tells the ECM, on the engine, that the brake pedal is depressed.

 It may not have anything to do with 12 volts DC ,at all. 

 It might just be "closing a circuit". "opening a circuit"

Hopefully, Bern will read this thread, and add more info.

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Edit: I am a little unsure , if this is the correct electrical engine schematic, for your engine. I do know it is for a CAT engine, that has a 70 pin design, ECM on it.{found photo of a schematic, on a PTO installation blog}

Notice what it shows for the brake pedal = closed circuit means brake pedal released. ( So, I assume, you must have a sensor, that is open, or broken wire, or some air pressure is telling OEM sensor to "open up" = thus it is "telling" ECM that the brake pedal is depressed)

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(On-Highway Engines Electronic PTO Installation and Application Guide3_70 pin_service brake pedal circuit _125.jpg)

On-Highway Engines Electronic PTO Installation and Application Guide3_70 pin_service brake pedal circuit _125.jpg (40KB - 1163 downloads)