The most accurate placement is pre-turbo aka exhaust manifold. I did the A piller set of three. Boost, EGT and Trans. Installation wasn't hard just took quite a bit of time to get everything right and factory looking.
When you drill the exhaust manifold make sure you use sharp drill bits. TAKE YOUR TIME and start with a small bit. Once you get a pilot hole with the small bit step up to the next size. Yeah this takes longer to do but it helps to cut down on the shaving size and helps to cut a perfect hole for tapping. Once you have the hole ready for taping use a flexible magnet to clear any shavings from inside the manifold. Once you have cleared the shavings use a shop vac to pull any other shavings from in the manifold. (I did this several times).
Now tap the manifold and again clean the tapped hole with the magnet and vac. All is good from here.
If you have a truck with an automatic you can route the wires through the hole where the clutch cable would have been run. There is a large plug covering the hole (you can pull it off with out use of tools via the engine compartment) and you can drill the center and cut to the lower edge allowing easy threading of the wires plus you can pick up some wire loom from Radio Shack to protect then and seal the hole from the elements.