Initially the controller was using the 5V supply from the arduino. This turned out to be a problem as the little controller was struggling to supply the necessary current to all components. The solution was install a 5V regulator that gets it Power directly from the cars 12V supply. This little component will get hot if the relays draw a lot of power. Make sure to give it some kind of heatsink if you expect multiple relays to be active for extended periods of time. Mounting it to plastic, as shown in the picture below, is generally not a good idea but in this case it was okay since at no point during operation more than 4 relays will be active for extended periods of time.
|5V Regulator with "Heatsink"|
See the Logical NOT operator? this is because this relay module has its outputs inverted. So a high level input (Logical 1) would mean the relay is off. So setting the output on the arduino low would actually turn on the relay.