This is a common problem. The wires that go from the connector under the seat and thread into the seat cushion are apparently routed against a support rod that causes the wire(s) to fray and eventually break. This can be repaired if you are handy. I have a PDF that came from the FB BMW i3 WorldWide group that documents the repair but I don't know how to post it here.
It could also be the THERMISTOR (basically a thermostat) that is is connected to those wires and it can fail. If it does fail, the fix is fairly simple if you are handy and might cost around $10-$20 for a replacement.
Or you can buy the entire seat bottom as a replacement with all the wires new and just replace it. Cost, I don't know $100-$200
I've yet to see anyone claim it was the dash switch or anything else. Until you get around to finding out the issue maybe get a small 12v heated blanket?