I own a 2005 Chevrolet Avalanche 1500 5.3L V8 Z71. After some really bad misfiring and shooting the parts cannon at it and a few mechanics later we figured out the problem is the engine wiring harness. Mechanic ended up getting a few wiring harnesses that didn't fit. I've been trying to find the part number online, but having a hell of a time. VIN 3GNEK12Z15G277509. If anyone can help I'd really appreciate it.

Welcome to the forum.
It would be best that you contact a dealer with your VIN#.
GM'* parts system will be accurate, & they will be able to provide you with the correct part number.
Using a good shop scanner, what were the reported OBD2 problem codes ?

Codes are random misfire and O2 sensor codes.

When you mentioned the parts cannon, I assume you replaced everything from spark plugs to the 02 sensors.
Was a compression test performed? If so, were the psi readings all within specs?

Spark plugs and wires, O2 sensors, MAF sensor, Fuel injectors, valve gaskets, computer etc... pretty confident it'* the engine wiring harness as they taped it up and it ran well for a few months, but it'* starting to act up again.

They taped it up ? Were wires damaged some how? How many miles on your truck?
I would hope the shop also would have scanned for coils firing correctly as well, Otherwise it could be a computer or module/ ignition issue.

Yeah they ended up getting the wrong wiring harness multiple times. They said they finally found the right one but the only one they could find was for $1,300. Which I definitely wasn't going to pay for a wiring harness. They said they couldn't find any damage to any of the wires, but they could try to retape it.

I already replaced the computer and if some coils weren't firing then the misfires would be specific to certain cylinders and not random, correct?

Wow- If I understand this correctly they cut wires and installed the wrong wiring harnesses?
They should take some responsibility for this, and credit you back over this mess.
I feel your wiring pain & suffering$. ++
IMHO call the dealer get the correct part number & get the correct harness installed..
Do you know anyone at GM or someone that buys lots of parts from them? I get 15% over cost at GM..that is what you need !!

Yes the misfires would be consistent,at a particular cyl- random dont always trigger a code right away, Its all about reading the shop scanners etc is there an ignition module separate were the coils tested properly?