I have different units than you, but had a similar problem with my bow-mounted 898 and my Terrova 80. I got the interference kit from HB but it didn't solve my problem. Called Minnkota and they sent me a wire with an inline fuse to connect between the negative of my trolling motor battery that goes to the Trolling motor and the negative of my starting battery.

Problem solved. The little fuse blew one day and the problem came back, changed the fuse, problem went away.

As someone mentioned, every situation is diff't, so what solves one may not solve another.