Polarity reversed!

[Lanark]

When measuring pin outputs to the ECU and dash, I read -12V?

Checked the battery and found I had voltage between the -ve terminal and the car body.

What is going on? How can battery polarity reverse?

I have been having starting problems so expect these are linked. Could this have fried my ecu ?

[Vultch]

As far as I can remember both Surf batteries are in parallel to give more starting current for cranking in low temps, nothing exotic.
The voltage difference between the neg of the battery and the chassis might suggest a bad connection, what voltage did you read?

When your measuring voltage on the ECU, did all the pins read neg voltage or did some read a plus voltage?