How to use communication protocol for a electric car with uart serial rs232

The tutorial was a great help. I succeeded to use the startmarker and filter the code. Only i'm not a champion in programming. So i still have two problem now. First i want to use the start of message and the number of bytes both as the startmarker i tryed to put them in a array but it didn't worked. Next i tryed different ways to count for the 19 bytes for the endmarker. But here i didn't succeeded eigther. Tommorrow another day for succes but if you guys have a tip great thanks.

AC 13 301B 00 003436 0F5B0F6E0F6A001F0065003E5E

0xAC: start of message

0x13: number of bytes to follow

// Example 6 - Receiving binary data

const byte numBytes = 32;
byte receivedBytes[numBytes];
byte numReceived = 0;

boolean newData = false;

void setup() {
    Serial.begin(9600);
    Serial.println("<Arduino is ready>");
}

void loop() {
    recvBytesWithStartEndMarkers();
    showNewData();
}

void recvBytesWithStartEndMarkers() {
    static boolean recvInProgress = false;
    static byte ndx = 0;
    byte startMarker[] = {0xAC, 0x13};
    byte endCount = 0;
    byte rb;
   

    while (Serial.available() > 0 && newData == false) {
        rb = Serial.read();

        if (recvInProgress == true) 
        {
            if (endCount != 19) 
            {
                receivedBytes[ndx] = endCount;
                ndx++;
                endCount++;
                if (ndx >= numBytes) 
                {
                    ndx = numBytes - 1;
                }
            }
            else {
                receivedBytes[ndx] = '\0'; // terminate the string
                recvInProgress = false;
                numReceived = ndx;  // save the number for use when printing
                ndx = 0;
                newData = true;
            }
        }

        else if (rb == sizeof(startMarker)) 
        
        {
            recvInProgress = true;
        }
    }
}

void showNewData() {
    if (newData == true) {
        Serial.print("This just in (HEX values)... ");
        for (byte n = 0; n < numReceived; n++) {
            Serial.print(receivedBytes[n], HEX);
            Serial.print(' ');
        }
        Serial.println();
        newData = false;
    }
}