1992 Toyota Camry Heater blows hot and cold

Hello, I am working on my mothers car this week. Her problem was that her heater would not get hot. and blew luke warm whild driving and cold at Idle.

I got a thermostat and replaced it. Now while I drive it the heater is blowing real hot, but when I stop and idle it blows luke warm.

Can you halp me figure out what is going on with the heater issue?

Thanks in advance!

Jeremy
in Washingtion state