Packed cars are a drag. A heavy-duty roof cargo platform gets the bulky gear, coolers, and duffels off your seats and up top where they belong. You get your legroom back, but you also take a hit on gas mileage and wind noise once it's up there. The best time to buy is early spring, right before the summer road trip rush hits and retailers start bumping prices back up to retail.
Don't mention financing when you talk to the dealer or retailer. Negotiate the price of the platform and any mounting hardware or locks as a single out-the-door total first. Once you have the final number, you can figure out how you want to pay for it.
Remember that the intro window is a deadline, not free money. If you still have a balance left on the Citi Simplicity® Credit Card when those 18 months are up, you will get hit with interest on the remaining amount, so make sure your monthly payments actually finish the job.