If you know how to remove the thermostat, cut the internal out with pliers and stick it back in and see what happens.
Or before you cut it try and boiling the kettle and pour boiling water over it in a cup and see if it actually opens.
Got myself an 02 wrx was pretty much standard with headlight conversion and ninja mid and backbox.
I have put a 3 inch decat pipe on form turbo to mid section and installed the 3port mac solenoid, removed the plasic y section and replace with hard pipe kit.
Now needs a map, what do you lot suggest ?
Thanks
Adam