I tried to un-string a tailgate loom properly a few months ago.
It would probably have worked OK on a nearly new one, but, over time, the loom covering becomes very sticky and it fights you all the way.
I ended up cutting and splicing after getting too many cuts and bruises.
Re-joining is definitely the easier way.
Check the wire colours (they might be different on the new door) then cut, trim back insulation, solder joint and cover with heat shrink sleeves. Wrap with self amalgamating tape for a neat finish.
Poor girl. I see a lot of them, often aggravated by neck and first rib problems which affect posture and muscle strength around the shoulder. Over time it leads to inflammation and sets up conditions for a frozen shoulder.
(I'm an engineer who retrained as a chiropractor )