Lights flashing like crazy is sign of dead battery. charge that puppy and it should start. As for harnesses under the seat. Theres one for the seat that comes out in between the vents. Then you will find some thick harnesses from the radio system and nav running along the drivers side of the console "hump" going from front to back then around the edge of rear seat and up the side to the rear. I dont recall seeing a harness on the floor directly under the drivers seat, so if thats were you mounted your device you might be in the clear.

Getting a visual check on where your screws went would be the best but it dose sound like your battery is dead and needs a charge.