Where do the OBDII port wires go to, BCM or ECM or both?

Hello,

The ECM and TCM communicate directly to pins 6+14 at the OBD connector.
The BCM communicates via class 2 serial data to the OBD connector on pin 2.

See image 1+2+3 below.

BCM wiring information for connector X1, see image 4+5+6.
For connector X2 see images 7+8+9.

ECM wiring information for connector X1 see image 10+11+12.
For connector X2 see images 13+14+15.

Please let us know if you need any more information.

Cheers, Boris

Hello,

The Data Link resistor is a 120-ohm resistor wired into the GM LAN communication bus to complete the circuit.
The other terminating resistor is installed in either the ECM or TCM. As both 120 ohm resistors are installed in parallel the total terminating resistance between pins 6+14 at the OBD connector should be about 60 ohm.

See typical locations of the Data Link resistor depending on your vehicle configuration.
Images below.

A link:
https://www.stockwiseauto.com/acdelco-88988999-data-link-resistor?Year=2008&Make=Chevrolet&Model=Silverado%201500

In image 4 you can see what the resistor looks like

In theory should be able to splice this resistor into the GM LAN wiring near the OBD connector if there is none fitted to the vehicle.
I would suggest to first check the terminating resistance of the tan and tan/white wire (pins 6+14) with everything plugged in. If you have a total resistance of around 60 ohm then no resistor is needed.

Cheers, Boris