Rally Dash Wiring Harness Questions

(3) The dash cluster wire coming out of position #10, teethe temp and fuel feed, that gets tied into the thicker (10/12 gauge) Black(Ignition)-Pink(Signal Light Flasher)-Black(Fuse Box) line that is coming from Bulkhead position W??? Does it matter where in the line it goes??? Does it matter an 18 gauge wire gets connected to a 10/12 gauge???

Going to have to look at that, I have to get to work soon