Walmart offers the Trademark Electric 12-volt Automobile Blanket in Plaid for $17.99. (Amazon has it for the same price.) Opt for in-store pickup to avoid the $4.97 shipping fee. That's the lowest price we could find by $5, outside the offer below. It features an 8-foot cord and measures 43" x 59".
A close price: Staples offers it in Plaid or Red or $18.99. Staples Rewards members bag free shipping. (It's free to join.)