Ok, sorry. Since the tacos were something I wasn't too excited about I totally forgot about the this thread.
I make the sauce and gave it to 5 different people to compare it with the king taco, I bought a cup of it for the test and all 5 said it taste the same...so the missing ingredient was...

CAJUN SEASONING.
CLOVE plays a main role too in this sauce.

The beautiful red color comes from the california chiles not the arbol chiles (which you still need in this sauce)

I nailed it!!!!
You won't believe what is the ingredient you were missing.... I report later gotta go to work now : )

I was intrigued by this topic and I am here at 9:00 am at the KT. Sauce is indeed something special, but I lived in Sonora and I don't sell my soul to any tacos. KT meat is really bad. Tacos are very poor quality. The Sauce like I said one of the best.
One thing for sure, They don't spend a lot of money making this sauce.

I am here at the KT eating this awful taco with this awesome sauce and I am thinking..

Clove FOR SURE!!!
Maybe Curry
Maybe Cumin but just a liittle bit...
Oregano
Pepper
Lime
Not too much onion
Not too much Tomatoe
California chile
Chipolte FOR SURE!!!
Chile de Arbol

THE RED COLOr is from either food colorant or canned tomato,

I wil give it a try...but I can tell it wont be easy... see what happen...