If you feed off the motor, the light will full brightness at full throttle and dim from there, which will look odd.

El Mod programmed a brake light to come on whenever you drop the throttle to neutral on their Kettenkrad system, which is neat, except it doesn't stay on the whole time it's stopped.

Best thing would be to either leave it steady on when the tank is on or use the servo switch recommended above to switch it off. The Tamiya M4 has a lights that are triggered with a stick/trim adjustment, but I think I used it a handful of times in 10 years.

Straight LED is simple to wire, the longer leg is positive, if you have it wrong, it just won't light. The resistor can go in any direction on either side. The more LEDs in series, the less resitance you need and there are plenty of calculators for it online.