I think you are right, apart from the stock part. In a just in time production environment each seat will be made to order and delivered to the factory by a supplier within a very short time frame of being fitted to a car. This keeps working capital to a minimum, shortens the time between taking delivery and realising the revenue and down streams storage costs to the suppliers.
To my mind there will be a unit cost increase per vehicle, extra wires, heating pads etc. But a reduction in configurator development costs and reduction is the number of SKUs reducing ordering errors and therefore reworks. I expect it will still be cost negative (I.e. more expensive) but this will be made up for from the upgrade or subscription cost.
The clever part for me is the subscription model. They say you can buy and keep the upgrade for say £150, but you can subscribe for £15 a month. People will think “I won’t use them, except for in the winter” so will subscribe instead of buy, then forget to cancel and end up paying £180 for 1 year.
I mean, how many people subscribed to Netflix and then didn’t use it after the initial period then cancel a year later £120 out of pocket?