35d has hydraulic steering not electric assisted right? And if I recall you can't get the active steering option either.
I would have guessed steering position sensor (related to faults) but that shouldn't impact it physically. Any physical changes (like wheel stuck at an odd angle or heavier then normal) should be related to the pump or the rack since it is a mechanical system.
Have they given you any explanation as to why they think its an electrical issue and not a mechanical one?
Sounds like the only thing they've attempted to replace is wiring which is strange to me.
Sorry the only relative experience I have is with my CTS-V, I would get "stability control engaged" messages when my steering position sensor was failing because the car would sense that it was going straight when it though my wheel was turned. It never impacted the steering feel though.
Herbert - '12 X5 35d / Sapphire Black / Black Perf Nevada / Aluminum trim / ZPP / ZPS / ZCW / ZAV / Apps / 3rd row
Betty - '04 CTS-V / Black Raven / Black Leather & Suede / 6 speed LS6 combo 400 hp & 400 tq
Sterling - '06 Silverado HD / White / Grey Leather / LBZ Duramax - Allison 6 speed combo / Crew Cab Short Bed LT3