Rebuild the “connected car” that the dead OEM cloud + TBox used to provide, self-hosted:

• Remote preconditioning — heat/cool the cabin before you get in (the headline feature).
• Remote telemetry — SoC, range, charge state, doors/locks — to your own app / Home Assistant.
• Better in-car UX — real turn-by-turn navigation and real speed-limit warnings on the driver cluster; climate controllable in parallel with normal head-unit use.
• Give the head unit internet again (the OEM data SIM is dead).

Remote preconditioning means a “start climate now” command must reach the car any time it is parked and off. So something with a cellular link has to stay reachable 24/7 — a permanent power draw that is a requirement, not something to design away.

• The 12 V battery is small (days of parasitic draw) and must never be deep-discharged — the car needs it to wake and close its HV contactors.
• The HV traction pack is huge (~63 kWh). So treat the 12 V as a buffer and the HV pack as the real energy source: monitor 12 V, and when it sags, wake the HV system so the DC-DC converter recharges the 12 V. This sustains an always-on load essentially indefinitely, bounded by an HV state-of-charge floor. (This is how the OEM TBox survives long parking.)
• Key synergy: the “wake HV” primitive is the same for preconditioning and for 12 V recharge — one result enables both.

• Tier 1 — always-on low-power node (e.g. ESP32/STM32 + LTE-M/NB-IoT modem, on the CAN bus, with a wake-capable transceiver). The TBox replacement: permanently reachable at single-digit mA, measures 12 V directly, monitors pack SoC over CAN, wakes the HV for preconditioning or a 12 V top-up, and wakes Tier 2 on demand.
• Tier 2 — heavy peripherals, powered/woken only when needed:
  • A companion Android tablet (own SIM + internet): computes navigation, acts as a Wi-Fi hotspot to give the head unit internet, and issues climate commands.
  • The head unit (these notes): executes climate/AC commands and renders turn-by-turn + speed-limit warnings on the cluster panel.

• 12 V low-voltage cutoff — stop the load / alert before stranding the car's wake logic.
• HV-SoC floor — stop recharging so the traction pack is never flattened.