Absolutely, even if you are making changes.  Its better to start with a clan reproduction factory harness and modify it from there.  You can add an auxiliary fuse block if you need more circuits.  All the aftermarket "hot rod" type harnesses require you to terminate EVERYTHING.  Royal PITA, and by no means "painless". 

I’m in the middle of installing a “Painless” harness in my ‘65 fastback. I bought the harness in 2007 and listened to one of my Hot Rod buddies before I discovered this forum. As TKO mentioned, the only thing terminated is the headlights. All the wires are marked but the instructions are vague. There is a four part video on You Tube that’s a little helpful But if I had to do it over again, I’d probably go the factory route.

Last edited by Mark in CA (8/28/2018 10:05 PM)

I purchased all my harnesses from Mustangs Unlimited, and found the quality to be as good as original.  I replaced all the wiring from the taillights to the headlights.  The only portion of the harness I've actually modified is the engine gauge feed.  Otherwise I've just supplemented by adding some additional wires as needed and using some tricks to make it all blend together and appear factory.  My method has been to modify only the small harnesses that are relatively cheap and easily replaced if I ever decided to go back to 100% stock.  When I added aftermarket gauges for example I simply modified the cluster harness to work with the new gauges.  It stays plug and play, and there was no need to hack up an underdash harness costing over $600. 

I would recommend at a minimum that you add relays for the headlights. 

Well Mike,  it all depends on what direction you are going with the vehicles. If you want to stay stock go with NPD's offerings or refurbishing the originals by Randy, previously mentioned. These are mostly plug and play and the easiest fastest method.    <http://midlife66.com/harnesses/ >

 If you plan on adding a lot of electrical accessories, pwr windows, door locks,  A/C, fans, EPAS,  battery relocation, etc,  go with AAW model and year dedicated rewire kits. AAW is not plug and play and a lot more work.
 I used AAW and was very pleased with the results, these kits will bring your wiring up to modern standards. Their product support and quality is second to none.
 AAW does not support the O/E externally regulated alternators on the early Mustangs, you have to go 3-G or one wire.

 As far as Painless is concerned, from what I have seen and heard  - - -   they are not

Last edited by Rudi (8/29/2018 8:54 AM)

this place makes most of the harness's for the retail houses.  They can custom make anything

https://www.ynzyesterdaysparts.com/

Rudi wrote:

Well Mike, If you plan on adding a lot of electrical accessories, pwr windows, door locks,  A/C, fans, EPAS,  battery relocation, etc,  go with AAW model and year dedicated rewire kits. AAW is not plug and play and a lot more work.
 I used AAW and was very pleased with the results, these kits will bring your wiring up to modern standards. Their product support and quality is second to none.
 AAW does not support the O/E externally regulated alternators on the early Mustangs, you have to go 3-G or one wire.

 As far as Painless is concerned, from what I have seen and heard  - - -   they are not

I have to reiterate what Rudi wrote, Merican Auto War iz Purdy dang good

Tubo
 

mustangermike wrote:

Such great suggestions guys. I’m also liking the sound of TKOs arguement. Go stock and then do supplementary wiring harnesses for aux stuff with a secondary block.

I agree completely with TKO.
 

The AAW 65-66 Mustang kit has the Original Mustang dash light & switch connectors Pre-wired and connected with the rest of the harness flowing from the new fuse box. This made wiring My 65 2+2 so much easier, in many cases subsystems I had added (like Power Windows) were provided for in the new harness. Thus saving the headache of figuring out how to and then modifying an original harness.

Tubo

I just finished installing an AAW kit in my 64.5. It is a very comprehensive kit, the directions and diagrams are excellent and it was very easy (if not a little time consuming) to install. Much to my surprise, when I hooked up the battery to test it, everything worked, including the glove box light. I would use an AAW kit again without hesitation.