If you use the same gauge wire with similar insulating properties, solder the splices and insulate the repair to keep water out then I can't see how they could void your warranty... They would do the same repairs as you would ...Now if your repairs cause the dealer time and parts in the future then sure they could dishonor the warranty in JUST that area... You probably paid a diagnostic fee so they shouldn't act like asses ...They sould not turn in a warranty claim as they did nothing in warranty... Repair the wiring and re-tape it like OEM ...The next dealer will never know it was damaged and would probably care less... You need to find another dealer...
Dave
Dave