Here is an easy way to thread wiring from the engine compartment into the interior of the car. No holes need to be drilled as this procedure will use the existing grommet that is below the pcm on the right hand side of the firewall. If you are considering putting in a manual fan switch or wiring for other relays under the hood, these instructions may be helpful to you.

I recommend that you read through the entire procedure before attempting any work. Use any information provided on this page at your own risk. This procedure and pictures are from a 1995 Z28. Other 4th Gen F-bodies will be the same or similar.

Although this deals with the engine compartment side of the firewall, you will need to remove the passenger side interior kick panel (4 phillips screws) and also the "hush" panel under the glove box (2 push pins) to gain access for threading the wire.

Remove the two 10mm computer hold down bolts. Wiggle and slide the computer up. The space is a little cramped. Take your time and do not scratch the paint on the fender. There is enough slack for you to set the computer aside without disconnecting it. click to enlarge
Down below where the computer is mounted, you will find a large grommet where wires enter the interior. The grommet is soft and will pull out of the firewall for easy access. There is a little bit of slack in the interior side of the harness, so you can pull some it into the engine compartment. You may also find a plastic harness retainer on the firewall side that, if released, will give you some more slack to work with. click to enlarge
Here is where you can make it easy on yourself. Make a "sewing needle" out of some stiff wire. I used some spare solid core copper electrical wire I had laying around. It is flexible, but still stiff enough to do what I need. Make a loop with your wire to form the eye of the needle. Slide the eye through the grommet, so that the loop is accessible. It helps to put a small amount of lubricant on the needle to aid in sliding it through (I used a thin film of Vaseline). click to enlarge
Pass several inches of the wire you are running through the eye of the needle.

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Pull back on the other end of the needle to easily pull the wire through the grommet. It will make short work out of fishing the wire through the grommet. click to enlarge
Merely pull whatever length of wire you need through the grommet to reach both sides of your connections. I find it easier to push the wire from the engine compartment side into the interior because the hole is hard to get to from the inside. You can make a "hook" to reach up and grab the wire from the interior side, once you have some wire pushed in.
Finish off your wiring with some corrugated tubing or route your wire through the existing tubing for an invisible effect. Place the grommet back into the firewall making sure that you seat the grommet groove for a good seal. Mount your computer back in it's place and you're done.
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This procedure is a perfect companion to route to the Accessory Power Feed that is located in the console of some Camaros.

Feel free to email me with any comments or suggestions.

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