Good idea, Sky have been using that strategy to sell upgrades for years e.g. a month's free subscription to Sky Sports and Movies. Offering a free month trial of AP will probably convert into quite a few extra sales. It's one of those features that you didn't realise you needed until you try it and then get hooked!
As for a discounted rate, that's what you get when you buy it with a new order. Otherwise I reckon a lot more people would buy the car without AP with a view to trying it later and then probably never bother. So I can't see that happening, except maybe if you buy it within say 12 months of the original car purchase. Basically get more of those people on the fence to buy in.