Based solely on my experience with my Classic roadsters Duchess, the fender mirrors bolt through fibergalss only…they are no where near any metal that is, in turn, connected to the chassis.
Your kit may be different. And if the mirrors do bolt through metal that is somehow connected to the chassis, then yes, you coudl run a short ground wire from the turn signals to the fender mirror bolts.
Paul Mossberg
Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
2005 Intermeccanica Roadster
If you own a TDr and are not in the Registry, please go to https://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)