I thought I had seen a related post on this from you before... thx to Mooseman for having the diligence to find / link them together.
I understand if you needed this vehicle to get to work, didn't have the means to transport it to another shop, etc. But, frankly (and I know this will sound harsh... sorry) ... the $2600 you gave them to rebuild it, after giving them $1100 and them failing twice before to correct the problem... wasn't the best plan. You now have $3700 CAD invested, and a transmission that works worse than when you originally brought it to them. From what I can see, a new (rebuilt) unit from GM costs $4000 or so (USD), at least as of a couple of years ago (I don't have access to their online parts network.)
Have you been to
acadiaforum.net ? While we appreciate you coming to us, there aren't that many here that have an Acadia (there's a couple, but most of the 'action' here is with that vehicle's predecessor - Trailblazer, Envoy, etc.) And our transmissions are nothing like yours.
It looks like the Acadia forum above has a LOT of info regarding issues with the 6T75e , especially (including some 'how-to' procedures.) Sadly, after glancing through their forums, it doesn't look like that trans has a particularly good track record. As good as GM's RWD transmissions have been over the years, their FWD transmissions (anything with a 'T' in the name) generally suck. Yeah, that's harsh, but I don't think you're gonna find too many fans of their transverse units.
Assuming you have no equivalent of the Better Business Bureau, consumer auto advocacy group, etc... you basically have three (main) options now, the way I see it. None are good.
1). Channel your inner American (not the gun-toting one... the one just below that one, that's indignant and pissed-off), and stand firm with the shop it's at now that you're not giving them one more loonie until this thing is fixed. That includes $ for the harness, if it turns out to be that. Mentioning that you've spent the equivalent of a manufacturer-rebuilt / warrantied unit may help.
2.) The above, plus (eventual) legal representation. Being American, I'm not well versed in Canadian Torts / Consumer Protection Law. But it might be time to speak to an attorney. Be warned that once the shop receives notice of filing, it will basically be 'war' between the two of you, and they will not lift a finger to help you from that point forward. Nor should they, having gotten it wrong 3x, now. The attorney will advise you on this. Basically, you're looking for: a) transmission that works, and b) court / legal costs, at a
minimum. If you've been spending money to rent another vehicle, etc. (or even if you have a second one in the family, but been inconvenienced), at least some of those costs should be recovered, too.
In the US, many companies use / force arbitration, and by using their services, you voluntarily give up your right to any other legal recourse. Check the fine print on your existing receipts, to see if this is the case with your shop. Then contact the attorney. (Optionally, you can infer to the shop that you're looking to pursue legal action over this, but only do this as a last recourse (see: 'war', above)
3.) Continue paying these guys to guess at your problem. This one might be the most frustrating, and the least satisfying. Basically, you're counting on their goodwill to take pity on you and stop charging you at some point.
There are other options, I'm sure... but that's the way I see things, given the info we have. If you take the Acadia to another shop, you'll basically 'lose' the money you've paid the first shop. (see: attorney, above... you may need to bite the bullet temporarily and pay another shop to fix the issues correctly. That cost becomes part of your pleading (filing) as well.)
Good luck, and let us know how it turns out. I wish I could be more help, but my working knowledge of GM's FWD transmissions is very limited. Do visit the Acadia forum, as I mentioned above. It would probably be a good idea to create a membership there and post about your problem as well... you're likely to get some good info about the 'real' source of the problem(s).