I bought a 06 tC at a local rebuild shop. It started right up while cranking but shut off with release of the key. It had a remote start at sometime. The blue and black wires were tied together at the ignition switch. I cleaned up the mess now it won't even crank.
The radio and heater work but the dash seems strangely void of lights when the key is on. Just a seat belt icon and a P for park. Unsure of what to think. A main power schematic would be of great help as I'm a electrician by trade.

Could the security system cause this?

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if it were me id rip the dash out and check the ECU connections and the rest of the wiring. I don't have a wiring diagram but id search for one if I were you. cant say id scrap the car because of that small issue.

 

Who ever had the car before you did a bad wiring job.

 

You can get all the wiring diagrams for your car at http://www.alldatadiy.com

It's not free but the level of detail is incredible.