After the blinker has been held on for while when it turns off if there is a bit of contact bounce it will latch the blinker for 3 seconds. its not a show stopper but Id like to rectify it. In reality it may rarely occur as the blinker switch contacts are a bit more robust than my dodgey piece of wire. Ive been playing around with some techniques to debounce it but nothing is really working out.

It may be that I rewrite without using interrupts and poll the input but ill leave it like it is at the moment if you want to have a play here is what I have so far.

/* Implementation of 3 second "auto" blinkers for SAAB9-5 by SAABAus Sept 2017 
 * uses pins 2 and 3 as inputs untill required than change to outputs to hold the blinker on.
 */

int Blink_left = 2;                     //  Left Blinker switch on pin 2
int Blink_right = 3;                    //  Right Blinker Switch on Pin 3
int debounceTime = 200;                 //  debounce lock out time
bool Blink_left_status = 0;             //  Store for the current status of the Left blinker
bool Blink_right_status = 0;            //  Store for the Current Status of the Right Blinker
unsigned long debounce = 0;             //  
unsigned long trigger = 0;
unsigned long Time_start = 0;
unsigned long OnTime = 3000;             // Time blinkers stay on for in ms 







void Right_Blink ()
{
  if (millis()- debounce > debounceTime)
  {
  if (Blink_left_status == 1)           //check to see if the left blinker is on
   {
    Blink_left_status = 0;             // if it is turn it off
    digitalWrite(Blink_left, LOW);
    pinMode (Blink_left, INPUT);
   }   
   else
   {
    Blink_right_status = 1;             // if the left blinker wasnt on turn on the right blinker
    pinMode (Blink_right, OUTPUT);      // set the right pin to an output
    digitalWrite (Blink_right, HIGH);   // turn on the right blinker
    Time_start = millis();              // Starts the 3 second timer
   }
   debounce = millis();
  }
}

void Left_Blink ()
{
  
  if (millis() - debounce > debounceTime)
  {
    
      if (Blink_right_status == 1)           //check to see if the right blinker is on
      {
       Blink_right_status = 0;             // if it is turn it off
       digitalWrite(Blink_right, LOW);
       pinMode(Blink_right, INPUT);
      }   
      else
      {
        Blink_left_status = 1;             // if the right blinker wasnt on turn on the left blinker
        pinMode (Blink_left, OUTPUT);      // set the left pin to an output
        digitalWrite (Blink_left, HIGH);   // turn on the left blinker
        Time_start = millis();             // Starts the 3 second timer
      }
    
   debounce = millis();
  }
} 

void Blinker_reset()                      // checks to see which blinker is on and turns it off
{  
   if (Blink_right_status != 0)
   {
    Blink_right_status = 0;              // Sets status to off
    digitalWrite (Blink_right, LOW);     // Turns off blinker
    pinMode (Blink_right, INPUT);        // Returns to listening to blinker switch
   }
  if (Blink_left_status !=0)
   {
    Blink_left_status = 0;              // Sets status to off
    digitalWrite (Blink_left, LOW);     // Turns off blinker
    pinMode (Blink_left, INPUT);        // Returns to listening to blinker switch
   }
}

void setup() {
  // put your setup code here, to run once:

pinMode (13, OUTPUT);
attachInterrupt (digitalPinToInterrupt (Blink_left), Left_Blink, RISING); //when the left blinker switch goes high call the left blinker ISR
attachInterrupt (digitalPinToInterrupt (Blink_right), Right_Blink, RISING); //when the right blinker switch goes high call the right blinker ISR
}

void loop()
{
    if (Blink_left_status == 1)           //this little section is for working out how to deal with the bouncing switch on release after the blinker has been on for a while
    {
    digitalWrite(13, HIGH);               // built in LED dipsplays the what the arduino thinks is the status the left hand blinker
    }
    else
    {
      digitalWrite(13, LOW);
    }
  
  if (millis() > (Time_start + OnTime)) { // if more than the alloted time has elapsed check to to see which blinke ris on and turn it off
      Blinker_reset();
      
      
  }
}