Thanks for getting back to us. You will need to download the service manual if you haven't already. It will have wiring diagrams, splice locations and connector pin outs as well as trouble shooting the DTC you have.
If you still have the siren/alarm hooked up under the windshield cowl, I would disconnect that before digging in any deeper than the C4 connection at the PCM. Flashing lights can be triggered by a bad/old siren which involves the anti theft system which can prevent the car from starting.
GOOD LUCK.