I'm not convinced it would be A51
If it is going to be a pin for a switch, it is most likely going to be a digital pin.
However, if it helps, this is the pinout.
case 31:
//Pin mappings for the BMW PnP PCBs by pazi88.
#if defined(CORE_AVR)
//This is the regular MEGA2560 pin mapping
pinInjector1 = 8; //Output pin injector 1
pinInjector2 = 9; //Output pin injector 2
pinInjector3 = 10; //Output pin injector 3
pinInjector4 = 11; //Output pin injector 4
pinInjector5 = 12; //Output pin injector 5
pinInjector6 = 50; //Output pin injector 6
pinInjector7 = 39; //Output pin injector 7 (placeholder)
pinInjector8 = 42; //Output pin injector 8 (placeholder)
pinCoil1 = 40; //Pin for coil 1
pinCoil2 = 38; //Pin for coil 2
pinCoil3 = 52; //Pin for coil 3
pinCoil4 = 48; //Pin for coil 4
pinCoil5 = 36; //Pin for coil 5
pinCoil6 = 34; //Pin for coil 6
pinCoil7 = 46; //Pin for coil 7 (placeholder)
pinCoil8 = 53; //Pin for coil 8 (placeholder)
pinTrigger = 19; //The CAS pin
pinTrigger2 = 18; //The Cam Sensor pin
pinTrigger3 = 20; //The Cam sensor 2 pin
pinTPS = A2;//TPS input pin
pinMAP = A3; //MAP sensor pin
pinIAT = A0; //IAT sensor pin
pinCLT = A1; //CLT sensor pin
pinO2 = A8; //O2 Sensor pin
pinBat = A4; //Battery reference voltage pin
pinBaro = A5; //Baro sensor pin
pinDisplayReset = 41; // OLED reset pin
pinTachOut = 49; //Tacho output pin (Goes to ULN2003)
pinIdle1 = 5; //ICV pin1
pinIdle2 = 6; //ICV pin3
pinBoost = 7; //Boost control
pinVVT_1 = 4; //VVT1 output (intake vanos)
pinVVT_2 = 26; //VVT2 output (exhaust vanos)
pinFuelPump = 45; //Fuel pump output (Goes to ULN2003)
pinStepperDir = 16; //Stepper valve isn't used with these
pinStepperStep = 17; //Stepper valve isn't used with these
pinStepperEnable = 24; //Stepper valve isn't used with these
pinFan = 47; //Pin for the fan output (Goes to ULN2003)
pinLaunch = 51; //Launch control pin
pinFlex = 2; // Flex sensor
pinResetControl = 43; //Reset control output
pinVSS = 3; //VSS input pin
#elif defined(STM32F407xx)
pinInjector1 = PB15; //Output pin injector 1
pinInjector2 = PB14; //Output pin injector 2
pinInjector3 = PB12; //Output pin injector 3
pinInjector4 = PB13; //Output pin injector 4
pinInjector5 = PA8; //Output pin injector 5
pinInjector6 = PE7; //Output pin injector 6
pinInjector7 = PE13; //Output pin injector 7 (placeholder)
pinInjector8 = PE10; //Output pin injector 8 (placeholder)
pinCoil1 = PE2; //Pin for coil 1
pinCoil2 = PE3; //Pin for coil 2
pinCoil3 = PC13; //Pin for coil 3
pinCoil4 = PE6; //Pin for coil 4
pinCoil5 = PE4; //Pin for coil 5
pinCoil6 = PE5; //Pin for coil 6
pinCoil7 = PE0; //Pin for coil 7 (placeholder)
pinCoil8 = PB9; //Pin for coil 8 (placeholder)
pinTrigger = PD3; //The CAS pin
pinTrigger2 = PD4; //The Cam Sensor pin
pinTPS = PA2;//TPS input pin
pinMAP = PA3; //MAP sensor pin
pinIAT = PA0; //IAT sensor pin
pinCLT = PA1; //CLS sensor pin
pinO2 = PB0; //O2 Sensor pin
pinBat = PA4; //Battery reference voltage pin
pinBaro = PA5; //Baro sensor pin
pinDisplayReset = PE12; // OLED reset pin
pinTachOut = PE8; //Tacho output pin (Goes to ULN2003)
pinIdle1 = PD10; //ICV pin1
pinIdle2 = PD9; //ICV pin3
pinBoost = PD8; //Boost control
pinVVT_1 = PD11; //VVT1 output (intake vanos)
pinVVT_2 = PC7; //VVT2 output (exhaust vanos)
pinFuelPump = PE11; //Fuel pump output (Goes to ULN2003)
pinStepperDir = PB10; //Stepper valve isn't used with these
pinStepperStep = PB11; //Stepper valve isn't used with these
pinStepperEnable = PA15; //Stepper valve isn't used with these
pinFan = PE9; //Pin for the fan output (Goes to ULN2003)
pinLaunch = PB8; //Launch control pin
pinFlex = PD7; // Flex sensor
pinResetControl = PB7; //Reset control output
pinVSS = PB6; //VSS input pin
#endif
break;