Just an update on what's been going on:
I moved the PSU to the front of the car about 4ft. away from Mac Mini, and the flickering has decreased. The PSU was originally in the trunk about 10ft away from mini. I recently moved it even closer to the mini, now about 1 feet away; the flickering now is worse than it was before.
I think that it maybe the connections on the power cables leaving the PSU to the mini. Perhaps my connections are bad that is why it has become worse now that it is much closer to the mini. Playing with the wires changes the frequency of the flickering.
Those of you who are experiencing issues, are you using the macpack or did you make the connections from psu to mac mini on your own? Because, right now it seems that getting a macpack may solve my issue. I have also noticed that unplugging the USB decreases the flickering as well, but that is most likely due to using poor wires.