Interference can encroach in 2 possible distinct paths - xducer/xducer cable or the power circuit ...

One should first determine which of these 2 distinct encroachment paths the interference is getting in ...

This is best determined by removing the xducer and the xducer cable entirely from the trolling motor and hang it in the water far away from the TM ...

*If the interference goes away = the interference is encroaching into the xducer or the xducer cable ...
*If the interference is still present = the interference is encroaching into the power circuit somewhere ...

It is less effective to apply interference remedies for one possible encroachment path if the interference is encroaching thru the other path ...

Determine the encroachment path and then remedies aligned toward that specific encroachment path can be suggested ...

Rickie