Dash Harness Repair

As far as I can tell, inside the car SAE "primary" or "general purpose" automotive wire is fine. Other than the heater and such, temperatures inside are normal and there is no oil or fuel. Marine wire is close to general purpose SAE although with finer wire strands which are tinned. If your in boating area, they sell it by the foot in bigger stores.
Rhode Island wire is reported to have a big selection of OEM matching wires for older and antique vehicles.
DelCity has larger and smaller (thrifty) spools in 5 or 6 colors.
https://www.delcity.net/store/Wire-&-Cable/Hopefully you don't have to splice in too much.
Since its out, identify what is bad and go from there.
I bought shrink tubing from McMaster mostly because they list the properties of the different types. Flexible, oil resistant, etc.