Best way to wire up to my head unit?

I'll be hooking up a new amp to the factory head unit of my Prius and I have two questions:

1.  What's the best way to make the connections?

I'll use butt connectors everywhere I can but my audio cables from the head unit to the amp will be 14 gauge.   Does anyone know what gauge wires are used on a Toyota wire harness?  Probably too small a wire to hook up with a 14 gauge using a butt connector?   But still a small enough difference in size to enable use of closed end crimp caps?

2.  Should I purchase a duplicate of the harness that's already there?

https://www.audiooutfitter.com/store/701761.html

On the 10 pin harness below, I'll be running new wires to pins 1,2,5 and 6.  So I'd have to transplant from the existing harness at most pins 3,4,7,8, and 10. 

Pin 8 is for an antenna 12v on but there is no power antenna so there might not be a wire but if there is I'll probably transfer it anyway. 

Pin 9 has no corresponding wire on the harness (no 12v-remote on) and so I'll likely have to tap into the wire on pin 3 anyway to run a 12v-on to the back of the car (either directly into the amp or to a PAC-OEM2 line out converter which needs it only if I'm going to use it to send a 12v on to the amp only when it senses audio present on one of the speaker wires.)

On the 6 pin harness, I'll be running new wires to pins 1,2,3 and 6.   Would only have to transplant wire to pin 5.

So is it worth it?  Nice to be able to connect the new wires to a new harness (and do so outside the car) but with so many wires having to be transplanted (that would otherwise remain factory connected) doesn't seem like the thing to do as I'll just end up having to do the same amount of crimping back there as I would just using the original wire harness.

Whether it's the old or a new harness, should I be considering attaching the wires directly into the harness instead of attaching them to the wires already part of the harness?  If so, how hard is that to do?

Thanks!

Continuing from above post:

Upon more investigation, the harness discussed above is not a replacement harness but is designed to plug INTO the existing harness; the wires on the end would then be connected to a new aftermarket head unit.  Not what I'm doing.

What I need is either an exact duplicate of the factory harness (maybe Toyota?) or an adapter which would plug both into the the existing harness and the head unit and also provide wires to hook up to for an external amp.   On PriusChat Someone has discussed building and selling his own such device.  Does anyone know if anything similar already exists?

Metra makes the Radio side harnesses (71-series) for almost every car made....  look up the harness on their site as if you  were just putting an aftermarket deck in and then just change the 70- to 71... and that's the one you need

heck i just looked it up for you... it's a 71-1761 and it will plug right into the OEM radio....

BTW, they also make a 60-1761 which is the T harness you asked about