Where do i put the thermostat probe in a corn snake wooden vavarium?

Put the probe through a hole in the enclosure (may need to drill/melt a hole) and use blu tac to block the hole up, this will hold the probe in place. Don't tape it down. Make sure the probe is resting over the heat mat, just on top of substrate is fine. You need a digital thermometer as well, place the probe of that by the one for the thermostat. Thermostat temperatures are just a guide as different heat sources are more powerful than others, set it low, give the mat a chance to heat up, check the thermometer, and adjust as necessary.

I personally put mine in the middle of the tank, about 3 inches above the aspen although I have two in mine, one at the hot side and one at coldish side. The temp should be around 32 degrees all through the tank.

you need one at the cold end and at the hot end (were the heat matt is) and should be set between 80 and 85 if adult and you wont notice heat coming off your heat matt but it will be doing and not to bad if it doesnt rise much as a snake mainly stays on the floor

You want it on your warm side about three inches above the top of your substrate. I would set it to 88, personally.

You stick it where the sun don't shine.