Generally, no, you should not install a bed stiffener on a lease car without explicit written permission from the leasing company. While a bed stiffener (or tailgate support) is a popular aftermarket accessory for trucks to prevent sheet metal flexing when hauling heavy, long items, it typically involves drilling into the vehicle's bed or tailgate. This modification is considered permanent damage by most lessors and can lead to significant charges at the end of your lease term. The core principle of leasing is that you are essentially renting the vehicle long-term and must return it in its original condition, minus normal wear and tear. Any unauthorized alterations violate the lease agreement.
The main risk is the lease-end disposition fee. The leasing company will inspect the vehicle for any unapproved modifications. Drilling holes to mount a stiffener is a clear violation. The cost to repair this damage—filling holes, repainting, and potentially replacing entire panels—can be substantial, often exceeding the price of the accessory itself. Even if you use a clamp-on or no-drill model, there's a risk of scratches or pressure marks that could be flagged as damage.
Your first and most critical step is to contact your leasing company or review your lease agreement for the specific modification policy. Some lessors might grant permission for certain accessories if they are professionally installed and reversible, but this is not the norm for modifications that alter the body. If you absolutely need the functionality, inquire about adding it through the lessor's official accessory program, though this will increase your monthly payment. For most lessees, the safest and most cost-effective approach is to wait until you own a truck outright before making such modifications.
