Originally, I wanted to use the massive hole in my firewall to run the harness to the area above the HVAC box like the original 85 ECM box was.
However, after realizing that the PCM was bigger than the space (05 and up PCM would be a better fit there) and I would have to extend most of the wires, I decided against that and came up with a new plan.
The new plan is to leave most of the stuff right where it was in the 2003 Tahoe; giving it a more factory look, rather than custom. Here's the basics I did. Your year might be different.
1. Since the 03 Air Cleaner was on the passenger side, I mounted it to the original 85 battery tray.
2. Then I moved to battery to the drivers side using a Diesel battery tray i got in the Junkyard. This also kept the 03 battery cables in place which was good, since the 85 didn't come with any.
3. The 03 fuse box will be reused and placed right next to the battery on the fender well. Again, because that's where is was in the Tahoe. It did necessitate moving the windshield washer jar.
4. For a W/W jar, I used a 2000-2005 Buick Le Sabre jar and mounted it under the original 85 passenger battery tray. It has a big body at the bottom and long neck that sneaks up between the 03 Air cleaner and the core support. (not pictured) Looks so factory.
5. The Tahoe PCM was originally mounted down low on the drivers side. So I built some brackets and used the original hangar to mount it in that same area to the Diesel battery tray. No extra wires were needed. If I would have gone to the core support, I would have had to extend wires. So that was out.
6. Why did I use the big 2003 factory fuse box? It always bothered me that the main power wires used fuse-able links. So, during the re-wiring process, I moved a lot of circuits to this box. I now have the two main power wires that go to the ignition and 85 fuse box each on one of the massive 60 amp fuses and relays. Also utilized the starter relay to eliminate full power going through the ignition switch; which can cause them to fail. Also used the fuel pump relay to keep things clean and tidy.
This has worked out well and during the mock up stage, the 5.3 turned on, pumped fuel and started just like it did in the 03 Tahoe.
Future plans will be to run the headlights, parking lights and fogs lights through this fuse box as well.
It did take a LONG time to wire it up but I am weird and love wiring.
I took the challenge and spent a long cold winter in the garage soldering each wire, one-by-one.
It was fun because my 13yo daughter was with me and she learned a lot. Still talks about how she found some circuits that I couldn't.
This will be her first car, so I want her invested in it.
The hardest part was doing it blind, without the factory 03 wiring diagram. I used most of the info from Brendon at LT1swap.com, google searches, AllData stuff from my local mechanic and sometimes just running continuity tests on each wire in the box!
All in all, I don't advise it unless you enjoy giving yourself headaches.
Here's some pics during the mock up stage. hope this helps.