[Pete]

Strip out ALL old wiring, even if it is not crumbling.
If you want to keep it stock looking along with the associated problems, get replacement harnesses and install them.

A much better method is scratch wiring.

Get an original copy wiring diagram for the model of car you have. The diagrams that come with new harnesses are not always completely right.

Get the QUALITY tools you will need to do the job. No Harbor Freight junk. You will need a GOOD volt/ohmeter. Used Simpson 260’s are cheap at $40 on ebay.

If you are not familiar with wiring cars, there are hundreds of books in the library on the subject. It is EXTREMELY important to know how to properly install lugs, solder wire and select the proper wire gauge for circuits.
Go with the books, they are more reliable than anything you will get from the internet.

Wiring from scratch is very simple if you think, “one circuit at a time”. Wiring from scratch is quite a bit cheaper. Wiring from scratch is more effective, will last longer and be more trouble free. The main reason being is you will run a separate ground return wire for each circuit. Using the frame of the car for return circuits is NOT good wiring practice. It just enables the car manufacturer to buy more yachts and golf clubs.
All ground returns are brought back to a common point close to the battery with a master shutoff switch.