Adding a tow package changes what your vehicle can do, but outfitting a car gets expensive fast. Labor rates spike in late spring right before everyone wants to haul trailers, so getting shop quotes in late winter saves real money on labor.
Negotiate the shop labor and parts price upfront before you think about how to pay. Once you have a firm price for the hitch and wiring harness, you can use a zero-interest card window to handle the installation cost in small monthly bites.
A zero-interest period is a payoff deadline, not free money. If you still have a balance after 18 months, regular interest kicks in on whatever is left, so set up automatic monthly payments to clear the balance on time.