Resurrecting this old thread, but I'm currently about to begin this process myself. I'm installing a Yaesu FTM-400XDR 50/20/5W dual-band transceiver with a Comet CA-2x4SR NMO 40" antenna using a Diamond Antenna K412SNMO articulating trunk lip mount off the passenger side of the engine hood with a flat braid grounding wire to the body. I'm still trying to decide where to place the main body but looking to place the controller on top of the dash, either centered or off to the driver-side corner. The mic attaches to the main body, which seems to necessitate not being far from the driver's seat unless I get an optional extension cable. I've identified the main grommet through the engine firewall under the steering column to run the power lines through but haven't identified if there's another option as I'd prefer to run the antenna coax line and power away from one another if at all possible.
As to the idea of using the seat pouch, that would not allow for very much airflow to the radio, which will heat up during transmission, so it would not be advisable, though it seems size kept that from being an option, as mentioned.
As to the idea of using the 12V outlet to power, the best option would be to run your radio's power straight back to the battery itself to avoid additional fuses and potential electrical interference. Using the existing 12V outlet or cigarette plugs should definitely be a last resort for optimal performance.
