If your talking about the plug for the block heater, it's located in front of the battery.
if you have one on your jeep it will be inder the exhaust on the side next to the oil filter.
Not unless you are in an area with temps way below 0F all the time.
I also have a 2000 jeep wrangler. This year has a problem with the heater core plugging up. You can either try to flush it out, or just replace the heater core.
cable
The heater inlet hose on a 1999 Jeep Wrangler 4.0L is located in the left, rear of the engine compartment. It connects directly to the heater core which circulates heat throughout the cabin.
Yes, jeep has used the same 6 cylinder engine for many years
Right in front of the battery.
In a 1997 Jeep Wrangler, the relay switch for the heater blower and motor is located behind the dash. It will be directly behind the HVAC knobs behind the dashboard.
Heater works fine in my Wrangler! Check the thermostat or the hoses or heater core might be plugged. The thermostat is easiest ^ cheapest to change so start there.
Heater core needs flushed.
Leaking heater core
Yes. It has a heater core in the dash that has air blown over it to heat the passenger compartment.