1998 Lexus seems to run cold and normal causing window fog up. Need a new thermostat?

Does the engine reach normal operating temp? Do you live in an extremely cold region? Are the heater hoses hot? Also, there could be a problem with temp door, under the dash.

windows fog up on the inside simply because of the hot breath of the occupants
this heat is cooled by the screen and condenses and so a fog is created
simple physics really - hot moisture + cold air =rain to over come this quickly run the air conditioner ( climate control) and that takes the moisture out of the air in the car and then the windscreen clears
you are going to say that it is too cold for the ac to run well at the same time run the heater until you have a pleasant temperature that is dry ( climate control--when it gets too hot the ac cuts in and when it gets to cold the ac switches off BUT the moisture that causes the fog on the windows is gone )

If your thermostat is staying open then it won't get hot in the cab,if the heater is bad,it will leak and fog up the window,another cause could be the blend door is stuck close,or the valve is stuck close and antifreeze can;t circulate thru the heater core.When you first start your car does the thermometer on dash go up then cone down,if it is then thermostat is good.

This is normal if your A/C doesn't work. When set on defrost the A/C kicks on and off to keep the fog from appearing. the fog goes away once the heat inside reaches a certain temperature. Unless the drain is clogged and the plenum is holding water. Do you have ice on the inside? clean the drain. It's on the passenger side, near the bottom of the firewall. A rubber tube coming out, that goes to nothing.

From your explanation it seems you are not loosing coolant so I would rule out a head gasket at this point. I would however check the "new" thermostat. Remove the thermostat and put in pot of water. Use a thermometer that will read the boiling point of water (212F 100C). Heat water and whatch that thermostat opens at its designed temperater. You can also see if the thermostat is opening by removing the rad cap when eng cold - start you eng and see if the coolant flows in the rad before the dummy light comes on. If the coolant does not flow your thermostat is stuck or improperly installed. Also be sure that your rad cap is of the proper number of LB rating for your vehice. The steam from your exhaust is normal at cold start up - it is simply condensation evaporating. That is how you can tell if a person has not let their vehicle warm up. If you follow them it will stop after vehicle reaches operating temp. Please let me know if this helps.

sorry to say but sounds like its time to replace heater core .....here is a really scientific test though....LOL.....let it get warm to where the window fogs ....with a clean finger wipe the window and tast test if it hase a sweet taste to it u need a core .......hope it helps

When you said you replaced everything, did that include the temperature sending unit?
The temperature sensor just operates the cooling fan or fans for the radiator.
The temperature sending unit operates the temperature gauge in the dashboard. If that thing is faulty, the gauge won't work right.
If you did change the sending unit, it could be possible that the computor is screwing it up, but in my 41 years as a mechanic I never heard of it, when a computor goes, nothing works.

thermostat stuck open poss

Yes. Unless you get fresh air you will fog up your windows. Your breath or rain on your clothes or snow on your boots will all cause the humidity to increase and this moisture will deposit on the non insulated glass. Kind,of like a glass of ice tea on a warm day you will get a condensation on the glass. The temperature difference is the cause. Try to run your A/C with the temp set to hot. This will act as a dehumidifier.
also check your pollen filter make sure it's not clogged.

chances are the heater core is clogged and wont allow proper circulation for the heater.i would suggest getting a new one put in and see if that dosent fix it for you.