Replaced motor and now car starts by itself when battery is connected

Hello,

Yes, it does sound like a bad ground or ECU issue. To be sure lets do a CAN scan on the system scanners cost about $40.00 on Amazon. Here is a video to help us with the ground cable diagrams and connections in the diagram below. Check the second diagrams where is shows 2,3 that is the connection I would check along with the the cylinder head connection.

https://youtu.be/InIlnsjOVFA

Check out the diagrams (below). Please let us know what you find. We are interested to see what it is. Here is how you replace the ECU as well. If you get a rebuilt unit give them your VIN number so they can program it for you.