'99 RX300 provides inadequate heat.

[sps513]

Hi All:

I have a '99 with only 157K miles on it, that has been in my family since 2001.  The heat is quite inadequate, and I am about to take it in to have the thermostat replaced, but it occurred to me that it's also possible that the heater core for the passenger compartment could be the problem.   
I'm pretty certain my dad never had the thermostat replaced, so since it's likely 25 years old that makes infinite sense to have it replaced.  I'm just wondering if anyone has had the passenger heater core replaced, as I have never had to do this on any car of mine in the past but I can understand how this could certainly be an issue considering the car's age.

Regards,

Steve

Cincinnati.

[RX400h]

Quote

The heater may stop working in Lexus RX due to¹:

• Low coolant level or air in the cooling system
• Clogged heater core
• Bad thermostat
• Faulty blend door actuator
• Bad water pump
• Dirty cabin air filter
• Bad blower motor
• HVAC control unit malfunction

If you have air blowing, but no heat, you could have an issue with the temp switch, or the antifreeze not circulating through the heater core. One user reported that they removed the glove box and lubricated both servo switches located behind it to fix the issue³.