There is an on-board power connector on the graphics card and the motherboard to provide power, so there is no need for an external power connector from the power supply.
(although the only Mac 6800 photos I've seen are tiny, it appears there's a stub connector on the card that (I suspect) uses the ADC (28V or 24V usually) DC power connector on the motherboard that's normally used for ADC display power to provide additional power (regulated down) for the 6800 card. That eliminates the need for Aux. (Molex) P.S. connector(s) like the PC/standard 6800 card versions have.)