It's 18 gauge I am pretty sure, but you could use 16 guage just in case. You should use wire rated for engine temperatures. I have used wires from late model junkyard 700 series. You should be able to pick up the red wire at the firewall connector. It is a grey connector for 8 wires.

Running the wire the same way as the original is a bit of a pain. It runs under the front of the block so you have to take the belly pan off. If you run it this way you should also put a cable protector on it. I would also run a new oil pressure sensor wire while you were at it.

I have also run a wire across from the passenger side without any problems.