Which factory wire do I tap for the ignition feed on a PSI standalone harness in a 1967 C10?

The harness ignition relay needs a source that has 12V in both start and run. We recommend the large purple starter excite wire picked up after the neutral safety switch on automatics, or the pink wire that originally fed the coil. For the firewall pass-through, trucks converted to automatic can use the now-unused clutch rod hole, which avoids drilling near a Vintage Air install.

Check this first:

• Use a voltmeter, not a test light, to confirm the wire holds a full 12V in both start and run (some original coil feeds are resistance wire and read about 9V)
• Confirm the wire is taken after the neutral safety switch on an automatic

Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead tuner (20 years of LS tuning). Every combination is different, so use at your own risk and check our safe tuning limits before any wide-open pass.