1968 GM Truck Factory Tach Wire Location

[Six862m6]

Hi all, been awhile since I last posted but have been a member here for over 10 years. Have a question that I'm hoping someone can help me out with. I have finally gotten around to neatening/cleaning up my '68 GMC's engine bay and am now starting to work on what is left of the wiring..... the previous owner(s) did some real questionable things wiring wise before I got ahold of the truck some 12 or so years ago and I didn't do much to the wiring under the hood until now other than making sure it was functional (which some of it wasn't when I first got the truck, such as having an internally regulated alternator physically installed with no signal wires ran to it and leaving the original regulator still plugged in.... safe to say it wasn't charging.).

I will be acquiring some new engine bay harnesses from American Autowire that will be correct for my truck's current setup (HEI, SI Style Alt on the passenger side, etc) as I am over looking at and trying to use what is left of the factory harnesses. That being said, here are my questions for you guys:

Can anyone here tell me/show a picture of where the factory tach wire location for a '68 (probably '67 as well) would be? I have already seen where it is located for the later trucks but their firewalls and firewall holes are slightly different than a '68. The harness directions are even 90 degrees different: 68's are arranged "vertically" and the later ones are "horizontal". My truck has what may be a factory tach wire already installed. It is currently to the lower left of the main firewall connection as you are looking at the firewall. I've marked it in one of the pictures that I have attached to this post.

Next, can anyone tell me what the grommet/hole is for that I have marked "what is this used for" in one of the other pics? Perhaps this was actually the factory tach wire location for 68's and the current one was just added by a previous owner?

In the same picture I have marked one other oddity as "WTH???". I believe this was definitely added by a PO, probably to control/power something that was added. It is no longer attached to anything inside or out (and wasn't when I purchased the truck) I have not seen this hole/wire in any of my searches, including in the one attached picture that is of another forum member's '68 GMC. However, you do see the item I labeled "what is this used for" on his truck. His truck being a 6 cylinder, I highly doubt he had a factory tach though.

Ironically, I used the factory manual choke cable hole to run the electronic choke wires through, (I was unaware of what this hole was for at the time). as well as the control switch wires for the fogs in my front bumper. I will of course be cleaning this up as I replace an rerun the rest of the engine bay harnesses.

Thank you in advance to anyone who can shed some light on these items for me!