Hi Art. Just saw your email.
To all, I build these and other harnesses on the side during nights and weekends around my day job and family commitments. So I don't have a lot of spare time for the forums, but am happy to help where/when I can. Phil (Harness Dr) and I work together, referring customers (trucks for him, suvs for me), etc. I've come close to standing up a website a few times, but worry that the flood of activity (per Phil's experience when his launched) will severely impact service levels. Each harness is hand built by me which can take some considerable time, especially the more complicated ones. The order queue's been pretty active lately and I'm very grateful for that.
I'll weigh in on the mirrors. In true GM fashion, although the circuit types are similar, adding factory DL3 mirrors to the trucks is different than the SUVs. Typically, the GM SUVs (Tahoe, Suburban & Yukon/YXL) will have all necessary wiring in the door harnesses for DL3 mirrors with the exception of the camera wiring. I have seen some exceptions (less than 5). The GMC SUVs will usually also have the necessary wiring in the body harness for things to be plug & play with the exception of the blind spot monitor. As previously mentioned, it also requires radar modules located in the rear bumper fascia. Tahoe & Suburban models that did not come with factory DL3 mirrors will be missing the puddle, turn signal and driver auto dim circuits at the connector to the door harness. The harness I build adds those circuits to the body harness.
For the mirror and window switches, it's dependent on whether the vehicle has the driver memory seat feature. If so, both the mirror and passenger window switch must be swapped out. This is due to how the components communicate with each other. GM routed a data circuit that tells the passenger mirror to fold through a specific window switch. If you were just to swap the mirror switch, only the driver mirror would fold.
If your vehicle doesn't have the driver seat memory feature, then only the mirror switch needs to be swapped out, but it's a different part number, more expensive and the connector is keyed slightly different. You can make the wrong mirror switch fit, but its pins are different and will blow a fuse as Art discovered.