Connect two-function brake light to three-function car circuit
I am going to assume you currently have three wires going to your old brake light, ground, Running, and Brake:

Running    Brake    Lamp

   0         0       off
  12v        0       dim
   0        12v     bright
  12v       12v     bright


The following circuit should accomplish what you are looking for using one LED:



When neither RUNNING or BRAKE are on, the LED is off.  When RUNNING is on, but BRAKE is off, 12v minus a diode drop for D1 goes through the pot R1 to reduce the voltage to the LED fixture.  You'll probably end up with a value of only 10 to 20 Ω, but I suggest starting with a larger value pot just in case.  Because of diode D2, there is no feedback into the BRAKE circuit.

When BRAKE is on, regardless of the condition of RUNNING, 12v minus a diode drop for D2 goes directly to the LED fixture. Because of diode D1, there is no feedback into the RUNNING circuit.