Hmm. Never used that, not sure what it's like.
Move your earth to the bolt on the isofix bar, just behind rear seats. Great place, just sand off a little paint around hole to get clear contact.
If you have some cable around. 1st test a direct earth to the battery to see if it resolves the issue. Just something rough to see quickly, this will help establish. But i'd put money on it being a poor earth.