Your choice of evaporator fan(s) for the interior is my least recommended solution. The bang for the buck just isn't there if you are truly a marginally gifted carpenter capable of a DIY install. You will get better performance from adding little computer fans to the back of your refrigerator.
However, if you are committed to the purchase and interior compartment approach because of the marketing literature then you can get power from the interior light bulb housing or pull a wire pair through the condensation drip outlet under the fins to the power takeoff at the back of the reefer.
Check out this thread for solutions others have completed. My reply is #8.