I've done two 130L Truckfridge installs on my camper builds. Both were vented with a source of fresh air outside and a vent inside AND an additional fan to suck air into the inside. Just finishing the 2nd build now. Here are some pics:
This is the fridge location. Note the small 4" opening. Since the fan on the fridge is only a 4", and the condenser coils are not huge, I'm not sure I see the point in a huge outside vent opening
This is a closeup of the vent opening with screening installed to keep the critters out
The original fan has directional markings on it. The air flow is so slight from this fan, it is hard to tell which way it is going. It sucks cold outside air INTO the compressor.
I bought this fan because it was a 4" squirrel cage style vs using any on the myriad of computer fans I have lying around because their is no room to vent it on the side like I did in my Hawk build. Instead, the squirrel fan sucks hot air from the compressor and blows it into the camper.
While it did move a huge amount of air, it was extremely loud!
Found a fan designed for use in home theatre systems. Super quiet, but nowhere near the air flow. Still way more flow than the stock fan though. This fan is designed to plug into a usb port (5v) instead of 12v. I added a cheap adapter, and am happy with the amount of air and the near silent operation. It is quieter than the original.
I did experiment to see if it would run on 12v, and it does, and blows a lot of air. Too loud though.
These are the wires I tapped into. They supply the original fan, and if the load of your additional fan is low enough (sub .5A) then it will work. More than that, and a protection circuit trips shutting the fridge down. If you still want to use a fan that draws more juice, you will need to add a relay. The advantage of using the existing wiring is that you don't need to add an external switch, and the added fan runs only when the compressor is running.