Originally Posted by montreal red
i am pretty sure you will loose the heated seat functionality when you do the switch. coding might help.
I agree that heated seat functionality will be lost. Regarding what you said... "coding might help"... help what? Help in restoring heated seat functionality to the seats that weren't heated to begin with? If so then I highly doubt that... unless all non-heated M3 seats are prewired for heating capabilities and have the heated seat elements already sewn into the seats.
Turboso... regarding your question about dash lights...
I did a heated seat retrofit project on my current 323i years back. I wound up having to buy heated power leather sport seats from a 328i owner local to me who was selling his car. The only dash lights that could possibly light up would be the airbag lights (possibly due to the seatbelt tensioners, etc. that I had to switch over from my old seats to the new ones). Fortunately for me my installers were professionals and were able to do the retrofit without encountering any error lights.