Smoke coming out of vents

SOURCE: I have a 2004 Jeep Grand 4.0 inline 6 with 116000

Smoke from that area is likely steam. It's possible that you have some slight seepage from the radiator or one of the hoses up front. At this point, the coolant loss is very slight and you may not see much in the way of dripping but there may be a wet spot on or near whatever is leaking that you can locate after it's been run and shut down for about a half hour.
You can also have a shop do a fluorescent
dye test to locate the problem.
Catching any leak while it's this small can avoid major problems later, so don't ignore it...leaks don't get better, they get worse.

I've noticed lately that many people are reading many of the answers given here and not bothering to note if they were helpful. don't neglect to do that as well!!!!
Good luck

SOURCE: thick white smoke coming from exhaust

I would think this will be a blown turbo.

These are fed by a small oil pipe and if the unit overheats (due to wear) - this oil fill feed directly into either the inlet manifold - or the outlet - exhaust.
If it is spilling oil into the hot exhaust - you will get a thick white fog. You can confirm this by placing a clear glass bottle or jar in the exhaust stream - and see if it gets a thick coating of oil on it. If it evaporates - it's water, if not - or feels oily - new turbo or overhaul.

Best get is sortd sooner rather than later - as you may be able to save most of it for an overhaul rather than scrap it if badly worn.

SOURCE: Gas Smell from air vents

i would look around the fuel injector themself be sure to use a flashlight for safety and go to an auto parts store and replace the o-ring or rings on the ones that is needed.

SOURCE: 99 Grand Cherokee fogs windows on warm defrost, but not on cold

Yes you do need a heater core, if not replaced it will only get worse.

SOURCE: 2003 Jeep Liberty has burning smell when stopped.

You need to find out what is causing the MIL (or CEL). Take it to an AutoZone or O'Reilly's and get the codes read. WRITE THEM DOWN, and report them back here.

I've got a '03 Jeep Cherokee, and one of the main issues I have had is bad evaporative emissions hoses. Chrysler used the wrong rubber formulation.

But, the burning smell could be a different issue altogether from the CEL issue, or may be combined. You will need to locate the source of the smell, and we can help you further.
I will say that if your not very mechanically inclined, you may have trouble diagnosing and making the repairs.