Here's how i got it to work on my S20 Ultra using ADB on Windows 10 .
Follow the instructions in the previous comments as to how to get your device into debugging mode .
Make sure you have no versions of Sygic on your device , make sure your device is in MTP mode for transferring files and then connect your device to the PC .
Download the Sygic app from this website then rename the file as app.apk , now download the ADB program "platform-tools" from the following link , unpack it to your Desktop then copy the now renamed app.apk file into this folder "platform-tools".
https://www.xda-developers.com/install-adb-windows-macos-linux
Open the platform-tools" folder then hold Shift and right click anywhere in the window and you will see "Open Powershell window here"
The first command you do is for getting your device connected to the PC .
Enter the command "./adb devices" , click OK to allow USB Debugging .
Enter the command "./adb devices" once more . Your device should now show as successfully connected .
Once your device is connected , do the following ...
Instead of inputting the info shown in the instructions from other versions of this app , add "./" to each line , ie "./adb push app.apk /data/local/tmp/app.apk"
then "./adb shell pm install -i "com.android.vending" -r /data/local/tmp/app.apk"
then "./adb shell rm /data/local/tmp/app.apk"
You should now find the app has been installed correctly on your device with Android Auto working . It did for me .
Hope this works for all those stuck like i was .
Cheers brassring for the upload of this great app
**UPDATE**
It was working great then Android Auto updated on my S20 Ultra , my Sygic on Android Auto then failed to work .
I have since tried several times to reinstall this with several different versions of the app to get AA working again but with no success
Hope somebody finds a foolproof way to get AA to work .