Reverse powering a 2G DSM ECU is bad a thing. This can happen when you try to jump start the car while having the positive and negative cables reversed, for example. This will almost always leave the ECU in a non-functional state. And, unfortunately, this often means the ECU is not reasonably repairable.
A quick check to see if the ECU might be repairable or not is to open the ECU case and check diode D103.
|Good diode (click to enlarge)||Blown & probably dead (click to enlarge)||Blown but maybe OK (click to enlarge)|
If the diode has managed to stay intact or the diode blew but the track nearest to it also blew, then you're probably one of the lucky ones and your ECU can be restored with a simple track repair.
If that diode is blown to bits *and* the track nearest to it appears to be intact, then the ECU is almost certainly not repairable.
NOTE: Just to be clear, the diode itself isn't the issue here. A blown D103 is just an indication that the rest of your ECU is toast. It is extremely unlikely that replacing *just* D103 will bring the ECU back to life. Otherwise this would be an easy fix…