On page 185 of the Richard Wright "Train 98 and 99 Coast Daylight" book, he states that a GP9 was used for power on train 99 on January 4, 1969. He also says that GP9s were confined to the commute pool before that, which is not totally true. I've seen photographs also of the Daylight and Lark with GP9s in multiple with other units in earlier years. One of the old SP Motive Power Annuals has a photo of an E9 and GP9 in multiple passing the Millbrae station on 99. The way he words it in the Coast Daylight book it sounds like it was a single GP9 ("....a GP9 was used as 99's motive power...."). It's not out of the realm of possibility. I believe that the winter consist of the Daylight was only about 7 lightweight cars by that late date, with the GP9s capability.