I need to control a 12vdc relay via 9 different sources. If I put a 12v diode on each of the 9 12v lines to the relay, will the diodes prevent back feeding on the other 8 lines/sources when only 1 line/source is active/on?

Yes, but you want to use diodes rated with withstand voltage greater than 12 volts to ensure currebt doesn;t fklow through them backwards. Also you want to make sure that either all 9 voltage sources have a commom ground, or else put a 2nd diode in the battery-return legs to prevent current flowing from one battery return to another.