Car dome light fading with arduino nano
You're not using digitalWrite properly. You should do:

if (digitalRead(domeLightIn) == LOW &amp;&amp; brightness != 255){


Just a sidequestion. What is pin7 connected to? If it's a 12v signal, you'd have to convert it down to 5v somehow. 

Also, the 12v in the car can be up-to 14v, and can have some huge spikes. Not sure if the sainsmart's regulator can handle those spikes. It might die prematurely. It's also just wasting 60% (generating heat!), turning 12v to 5v. I would suggest adding one of those cheap car-phone-chargers.