Carrying bikes on the roof changes how you use your car, but hauling gear around town also takes a toll on your gas mileage and suspension over time. Prices on rack setups dip when riding slows down for the winter, making it the smartest time to buy.
We need to separate the price negotiation from how you pay for it. Dealerships and specialty shops will deal more on the hardware when foot traffic is low, and you can sort out the financing details after you agree on the final number.
That 18-month window is a strict deadline, not free money. If you still have a balance left when the intro period ends, the remaining amount will start accruing high interest fast.