Upgrading your setup with a solid set of LED light bars and mounting brackets changes night driving completely. You get clean visibility on dark trails and back roads, but the initial hardware cost adds up fast. The best time to buy is late December during end-of-year clearance events, when retailers drop prices to clear out shelf space for next year's inventory.
You should negotiate the price of the hardware and any extra wiring harnesses before you ever talk about how you are going to pay for it. A financing window is just a tool to spread out the cost, not a discount on the retail price. Keep the bargain separate from the payment method so you know exactly what you are paying for the gear.
Remember that the intro window is a strict deadline, not free money, and any remaining balance left over when the 18 months run out will trigger standard interest charges on what you owe.