you use a 12 volt fan and can pick up the 12 volt power at the back of the frig. You can put a 120mm fan inside the vent on the roof pulling the air in from the bottom out side vent and out the top vent. Or you can mount the fan on the bottom pushing the air up the rear coils of the frig. For the bottom you can get a squarrel cage type fan that pulls the air in from the side and pushes it out the top. Most electrical supply places for radio equipment will carry the fans. Or check on line for 12 volt commputer fans.
Thats what I use. 2 computer fans at the top of the vent. Power was from behind the fridge and I put in a switch as well.