They never cut any wires and they were the 3rd mechanic I took it to. First 2 mechanics couldn't figure it out and they spent a lot of time on it trying to figure it out as sometimes it would run completely fine and other times it wasn't even drivable. They barely charged me for the time they spent on it.

Oh ^ that is good news then, at least that part.
The problem with electrical is understanding the systems and having access to shop scanner & the factory service manuals.
You can sign up to ALLDATA (single vehicle personal) and obtain the manual for your Avalanche.It has proper diagnostics, & electrical schematics..
The next shop should contain the words "Auto Electrical specialist"

Late to the party. If they were able to tape it to make it reliable, I think they should be able to get in there and do a professional repair with solder and shrink tube. If the tape repair was holding wire to plug/connector/etc., then it should be easy to get the same non-broken plug/connector/etc. with wires over a parts counter and/or in good condition from a junkyard . . . then solder/shrink-tube together.

These aren't known for chafing wires etc. I am really curious where it/they were broken and what caused it.

Personally, I can't imagine swapping out an entire harness if the breaks have been found . . . or in most cases even if the breaks haven't been found.

They said they couldn't find any broken or damaged wires, but they were confident the wiring harness was the problem. Must have been because them retaping it worked... for a while

Where did they re-tape?

The problems are likely right there.

The whole harness... that'* why I'm just trying to find a new harness. Sounds like the easiest way to me

They re-taped the entire harness? Like miles of tape wrapped around many many feet of wiring harness on the engine, around the engine, over to the computer, and into the firewall?

This makes little sense to me. Among many others, I've regularly worked on my 313,000-mile GMT800 YukonXL, a friend'* 330,000-mile GMT800 Avalanche, and a different friend'* 200,000-mile GMT800 Tahoe. Pretty much all of the GMT800s are high-mileage now, and I've never heard of one with a "bad harness". To me this is basic troubleshooting with basic wire issues, and possibly a mechanic that [doesn't like/isn't good at/doesn't want to/etc.] has planted the seed of needing to replace the vehicle'* wiring harness.

You can't get a harness at the salvage yard?

With what these hacks have done I suggest he replace the harness or take it to a shop that does good diagnostic and electrical work.

I agree.

At the same time I don't have a full understanding of what and where got taped etc. so hopefully Keithheidt comes back with more information.