An endearing term for the middle seat in a pickup/car. Usually the front seat, where the man has the ability to drive with his arm around his sweetheart. NOT a deragatory term, but usually said with affection. Mostly used by older people, who aren't completely consumed with sexual thoughts.
(As climbing into a pickup)
David: Did you remember to grab the directions?
Susan: I did. We start out by driving 2 hours South on I-5.
(They begin buckling their seatbelts, but David smiles and their eyes lock)
David: Care to sit in the sweetheart seat?
(Susan smiles and slides on over. He puts his arm around her shoulders and they set off.)