near the fire wall where the heater hoses enter
No, it is not required. You can remove it and splice the heater hoses together.
Check for a thermostat that is stuck open. Also can be a clogged heater core, or defective heater temperture control valve.
the vacuum hose connects to the airbox
Jeep transmissions do not have modulator valves
A 1999 Jeep transmission does not have a modulator valve.
It is the Positive Crankcase Ventilation (PCV) valve.
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.
Back of valve cover on pass side rite below heater hoses
Fixing the heater contol valve is very simple, however, what i might say could be totally different than what you need. I have a 1987 Jeep Cherokee Pioneer 4.0 and my heater control valve went bad and started leaking coolant like crazy. This is a very common problem with these older Jeeps. luckily, it's an easy fix. What you have to do is go to your local parts store (Schuck's/Checker/Krager, Napa, Auto Zone, O'Reilly, whatever) and ask for a heater control valve for your application (year, make, model). then it's just a matter of locating the valve (between the coolant reservoir and heater core) undoing a couple hose clamps, taking out the old one and putting in the new one. very simple. I hope this was helpful.
cable
It doesn't have EGR
Check the vacuum operated valve at the hose going to the heater core.