Brake Lights

You are positive they are the brake lights and not the running lights, Im not trying to insult your intelligence, just want to be sure. It sounds like you may have a brake light switch that has failed if it is the brake lights that are on. Or possibly a BAS relay that is stuck. It is located under the rear seat in the relay center. Try swapping it for another relay to test (Like A/C)

Is it a 2 wire switch? If so use a test light to see if the switch is passing power through it without the brake being pressed

Sounds like the brake switch is bad. You should be able to purchase it from an auto parts store, maybe the dealer

The switch for this vehicle is actually called a brake pedal position sensor(BPP), a three wire(BRN/YEL/WH) sensor that is an input to the brake control module. With the BAS relay removed(left rear fuse block), do the lights still come on? The EBCM supplies the control voltage for the relay at pin R1 based on BPP input, and a constant ground G402. If you still have brake lights(all three including high mount) with the relay removed, about the only thing you could have is a short to voltage on circuit 17 after the relay. Is there any aftermarket stuff added to the vehicle that could possibly tie into this circuit, is one thing to consider. Pin R3 of the relay is switched volts out to the lights. The schematic that I have, shows no other splices or anything that could induce power to this circuit. The fact that the cruise don't engage tells me that a brake switch input is sensed by the EBCM telling the PCM via data bus that the brake is applied. With the circuit intact, disconnect the brake pedal position sensor, do you have lights? If you have lights w/o the brake switch, you could also have a faulty EBCM. If the pins on the relay are labeled, check for voltage on pin R1 w/o the brake switch connected, if it is still hot, then I would say you DO have a faulty EBCM. Shorts to voltage are relatively uncommon, as they almost have to occur by the right wires shorting together in a harness. Shorts to ground can occur almost anywhere, but you're dealing with switched powers. If the left rear fuse block is a bussed electrical center, and you have brake lights with the BAS relay out, then I would say you have a shorted electrical center, rather than a circuit(17) shorted to voltage.

Some more info on operation, for what it may provide:
The brake pedal position sensor is a resistive device used to sense the action of the driver's application of the brake pedal. The brake pedal position sensor provides an analog signal that will increase as the brake pedal is pressed.

The brake pedal position sensor receives both a 5-volt reference voltage and a low reference signal from the electronic brake control module (EBCM). When the brake pedal is pressed, a variable signal is applied from the brake pedal position sensor to the EBCM through the brake sensor signal circuit. The fuse block left rear supplies battery positive voltage to the bass relay switch through the BASS fuse. One side of the bass relay coil is permanently grounded at G402. The EBCM supplies voltage to the bass relay coil upon receiving the signal from the brake pedal position sensor. The switch contacts of the bass relay close and battery positive voltage is applied through the bass relay switch to the center high mounted stop lamp (CHMSL), and to the turn signal/hazard flasher module through the stop lamp switch signal circuit. The ground for the CHMSL is applied from G402. The turn signal/hazard flasher module then supplies voltage to the left and right stop lamps in the tail lamp assemblies that have ground at G401 and G402. For export vehicles the stop lamps receive battery positive voltage directly from the bass relay.

This is a BPP calibration procedure that involves the use of a scan tool, I don't know if this could be the entire problem:
Brake Pedal Position Sensor Calibration Description and Operation
Calibration Criteria
Brake pedal position sensor calibration must be preformed after the brake pedal position sensor or electronic brake control module (EBCM) have been serviced. The calibration procedure will set the brake pedal position sensor "home" value. This value is used by the EBCM to determine the action of the driver applying the brake system and to provide this information to the vehicle subsystems via the class II communication bus.

Calibration Procedure
"Â Install a scan tool.

"Â Turn ON the ignition, with the engine OFF.

"Â Select Chassis.

"Â Select DRP/ABS/TCS/VSES.

"Â Select Special Functions.

"Â Select the BPP Sensor Calibration and follow the directions displayed on the screen.

I am not certain on the price, it is very seldom inexpensive being a safety item. The high mount should be on, unless it is completely inoperative. The way the schematic is laid out, all the brake lights recieve power from the same source at the same time.