The circuit seems simple just looking at the schematics, looks like nothing could go wrong. The problem is in what is not seen in the schematics, an issue that would rarely be encountered outside LED bulbs.

I had a 1994 Ford Aerostar 4.0L AWD. It had an overbuilt A4LD transmission that was upgraded to handle the heavy towing I do. I wanted my vehicle to be unique, so I had clear tail lights with LED bulbs. I ran a heavy duty flasher on the turn signals to address the rapid flashing, and nothing special on the brake circuit.

I had this setup maybe a year without any issues. Then one day while I was out of town coming back from a 350 mile trip (one way), the trans began to feel a little off, but it was difficult to place my finger on as I was traveling and highway speeds. When I got to surface streets back home, it was apparent the the shifts were delayed, and that the trans was sometimes starting out in second gear.

So off the the trans shop. Diagnosis, the transmission had been badly overheated, the seals were toast, the torque converter was toast, some of the bands had begun slipping because the valve body had been cooked enough that the pressure was being lost through basically melted rubber seals.

So $2800 later, trans gets rebuilt back to the same specs it previously had. They take it on a road test, TCC lockup isn't happening. Determined that overheat was due to the TCC lockup not working. So they test the solenoid and its circuits, they are fine, but are not being commanded on by the computer. Closer investigation reveals the computer is seeing the brakes as constantly on. So the brake switch is tested. Switch is working correctly. Test reveals that they is a constant 2v signal on the brake circuit when the brakes are not on (this is on the non-powered side of the switch). Turns out the brake signal is a biased circuit. The biased voltage is a low amperage voltage that is applied inside the computer. It may have been part of a brake bulb failure that was never implemented, or it could be normal diode leakage, anyway, many Ford models have the same design.

When an incandescent bulb is used, this biased voltage is not detected, the incandescent bulbs draw enough power to discharge this voltage, causing a 0v reading on the brake circuit. Disconnect all the brake bulbs, the 2v signal returns. The computer sees this low voltage signal as the brakes being applied. The LEDs draw low enough power that they do not discharge this circuit. My van worked flawlessly for almost a year because my third brake light was incandescent. But on that trip, the third brake light burned out.

The trans shop disconnecting the LEDs, and make it go away by using regular bulbs. By looking at the schematics, you would never know this is the case. So if I seemed harsh on that issue as well, its because I had a very costly experience. Who is to say if all other Fords are affected the same way? I doubt all are. But resistors are cheap, and can be bought from the same companies that sell the LED bulbs in many cases. Transmission controllers, traction control, AWD modules, ABS, all make decisions based upon the state of the brakes. Any of those systems could malfunction, possibly in unexpected and expensive ways. Hindsight is 20/20. Cheap resistor or $2800 transmission rebuild.

As far as what resistor to use, you don't use just any Radio Shack resistor. Many sites that sell LED automotive lighting actually sell special made resistors that approximate the load of a 5w incandescent bulb. I don't know off the top of my head what resistance. I'll just attach a link. 2 IN-LINE LOAD RESISTORS WITH MOUNTING PLATES 9005