Fried ECM
OK.
A couple suggestions.
But first, forgive me if you already know this, lets be sure the measurements are what we think.
(12v when checked to bat pos.)
When one of the test leads is on the battery positive terminal and the second lead is placed any point connected to the positive, the voltage should be zero.
When its not zero, we're measuring votlage drop from resistance to flow.
On the other hand, when one test lead is placed on the negative terminal, whatever the second lead touches is voltage above ground.
Voltage above ground is the normal assumption when not otherwise specified.
My first suggestion is to visually trace the wires on your car and make sure they are routed correctly and the terminals look good.
You can copy the schematic and either print it out or open it in MS Paint.
Check the wire colors and connections in my schematic against the factory service manual wiring diagram for your car and year. Each car and year is a little different.
Then you can do voltage checks if and see if they make sense for the circumstances.
When testing, check the ammeter as this will show whether current is flowing out of the battery (or into it for recharging).