Left wing.
We found that we had just the right amount of wire with the probe to place in the left wing lower skin just to the inside of the first inspection cover, then run the wire up to the panel with no splicing of the wire. By putting the probe there it is in the shade of the wing somewhat in the medal from the leading edge to the trailing edge. Far enough away from the engine and prop slip stream so as to be in fresh un heated air and directly into the air stream to get as good a temp. reading as was allowed. What works for us may not work for you as there are a lot of panel set-ups out there and you need to think through you system and it's style and your needs and wants.
Hope this helps, yours, R.E.A. III #80888