Inject the downloaded DLL into NeedForSpeedHeat.exe before you leave the garage (will not work if you wait until you're already in freeroam). You can use Cheat Engines' built in DLL Injector for this.
Go to Memory Viewer -> Tools -> Inject DLL and select the DLL you downloaded from here. When it asks if you want to execute the function of the DLL then click yes. Now it's running.
Press ALT + R to reset your auxiliary count back to 2. I've made sure that you can't do this during a race however :). Press the END key when you want to stop the mod. You'll hear a beep sound when you do that.
Special thanks to LianUnleashed for helping me test this.
- Fixed a bug where after switching cars when your auxiliary count was zero, you were only able to use your auxiliary once instead of twice.