Connect the minus side of your 12V supply to ground, then connect the plus side of the 12V supply to either PWR 1 or PWR 2.
One diode within the bridge rectifier will conduct and pass the voltage through (minus about 0.6V) while the other 3 diodes will do nothing.
Take care though - you will have lost your reverse polarity protection. Might be a good idea to add a fuse inline with the +12V feed to get some degree (not 100%) of reverse-polarity protection back.