USB C Car Charger Adapter with Voltage Display, 30W Cigarette Lighter Type C Fast Charge Power Delivery & Quick Charge 3.0 (PD & QC), Volt Meter Battery Monitor, Compatible with iPhone Galaxy

Maybe something like this, press start button without brake pedal should put it into accessory only mode I believe, would indicate voltage of the 12v system while not being dcdc charged off the traction battery. https://a.co/d/05DdrpDY

https://www.amazon.com/dp/B08N4WKJMT?ref=cm_sw_r_cso_cp_apan_dp_03NWDD2CWS4S9TH06237&ref_=cm_sw_r_cso_cp_apan_dp_03NWDD2CWS4S9TH06237 This one has a blue and red version. I appreciate that as a user friendly way to rule out possibilities, that don't require going back to the local trusted shop.