clogged heater core
Hot coolant passes trough the heater core, giving it warmth and the heater core distributes that heat to the cabin when you turn your your heat knob.
If the heater core is clogged, coolant can not pass and if it can not pass the heater core can't get warm.
To solve this, you must perform a heater core flush.
If the flush doesn't work and the heater core is super clogged, you need to replace it ( around 1000$, 90% of it is labour)
bad soldering connection on temperature change knob
In a lot of rx8's, the soldering on the temperature change knob is extremely faulty from the factory which causes no connection between the board and the knob itself.
To fix it, pull out your radio, disassemble the radio until you get to the bottom part where you can slide out the AC controller unit (with the board) and just solder 3 of the connections on the temperature control knob in the back.