How do you power a mobile rig in a Mach-E without hammering the tiny 12V battery?

The setup you are describing—running a separate lithium iron phosphate battery charged from the vehicle while it is operating—is definitely the correct way to isolate the radio from the main bus voltage. Most modern EV charging ports, especially in the rear cargo area, are designed to handle accessory loads up to a certain amperage, usually around 15 or 18 amps continuously, but a 30 amp LiFePO4 battery will draw far less than that while cruising, especially if you are charging at 10 amps. The cargo outlet itself won't complain because it's designed for constant trickle charging. As long as your charge controller is correctly set up to draw power safely and the boost conversion into the radio's 13.8V rail is clean, you're good. You won't see vehicle warnings unless you attempt to draw a massive, instantaneous surge, which a typical ham radio during transmission won't do. Regarding noise, isolating the radio on a battery and ensuring the power lead is as short and well shielded as possible is the best preventative measure. Since the radio is constantly drawing a stable load from the battery, you minimize the chances of picking up transient electrical noise generated by the EV drivetrain when the car is running. The noise issue is almost always related to the grounding and the length of the power run itself. For the control head, since drilling or permanent mounting is out of the question, you are limited to non-invasive mounts. One approach is using a bracket that clamps onto the door trim or the passenger seat and uses a weighted base to keep it from moving. If you want it to feel a bit more integrated, I found success using a custom dashboard shelf that slides into an existing gap and holds the radio while allowing full climate control and screen visibility. A small block of foam with a custom cutout also works if the radio is small enough to sit securely in place without falling. Don't bother with generic adhesive mounts; those won't last with the vibration and temperture swings in a car. Look for something with a secure, bolted clamp that secures it to a solid, flat piece of interior trim, not just a flimsy plastic piece.

I totally agree with the advice on using a seperate battery and isolating the power run. The noise issue is defintely minimized once the radio is running off a steady external source. For mounting, I had the same struggle with the interior angles. I ended up using a custom foam block I cut myself, and instead of putting it flat, I angled the radio about fifteen degrees up. That angle gave me better line of sight for the display and kept the antenna run from sagging, which kept the signal quality much cleaner than when it was lying flat on the dash.

I was looking at the noise angle, and it's always more than just the radio itself. When I finished my setup, I found the constant low rumble was coming from the wiring harness vibrating against the chassis. It wasn't the RF noise, just the AC hum. I had to run a complete section of heavy-duty cable loom around the whole power run, and that single material completely cut out that distracting sound. Makes a huge difference even if the power source is clean.

I appreciate all the detailed advice on the power setup. Running off a spare battery makes total sense to keep things clean. I'm still thinking about the cables running inside the car, though. Since the Mach-E is a tight space and I'm putting the radio in the back, how much extra wire would it take to get the power and the antenna signal up to the dash? Would running a really long run of cable make the setup difficult to hide inside the car?