2001 GC negative battery cable

in place of running a totally new molded cable, is there a good way to un-wrap from the harness the "negative" battery cable, cut it back to new copper, and crimp an extension onto it ?

Actually, you can buy a "generic" battery cable at any parts store that will do the job, the harness is just to make it easier on the assembly line workers.

The lug end that goes to the engine block will be mounted where it can be reached easily as they had to put it on after the engine was installed at the factory.

Disconnect it, roughly measure the length, and pick up on at the parts store.

Be sure to get one with the smallest guage number (wire guages are bigger diameter for smaller number sizes.) you can and consider doing the same for the positive cable.

Now, for routing it (them):

Just keep them away from rotating or moving parts, hot manifolds, etc.. Cable ties can be used to tie them in place but allow slack for engine movement.

Clean the cable ends (both ends of each cable) yearly at a minimum and you can use ordinary chassis grease or even vaseline to reduce corrosion.