DIAGNOSIS Input voltage is too low. The car's 12V accessory socket (cigarette lighter) often provides only 11-13V, especially with the engine off. The Explorer 1000 requires a stable 12V/24V input at high current for reasonable charge speed.
FIX Start the car's engine. This raises the alternator output to ~13.8V-14.4V. Use a heavy-gauge, short car charger cable designed for high current. The thin, long cables that come with many devices cause too much voltage drop.
IF THAT DIDN'T WORK The car socket's fuse may be blown or the socket itself may be corroded, creating high resistance. Test the socket with another 12V device. If the socket is good, the unit's DC-DC buck converter input may be faulty.
HEADS UP Car charging is inherently slow for a 1002Wh battery. Expect 10+ hours from 0% even under ideal conditions. It is for maintenance, not fast replenishment. For speed, use AC or solar.