Well, the bottom line is that a 90w PSU is 90w regardless of whether it's in the car or not. Does everything work when you plug in mains power (like an extension cord from your garage?) Cause if so, then the problem is with your invertor. I'm betting that your 150w invertor isn't actually 150w continuous, and is really rated more around 75w continuous, 150w PEAK... I'm betting you've already destroyed your invertor.