I have 2000jetta vr6, and actually just 90W power supply, absolutely no problem.
I ran all the wires all the way from trunk to dash, and actually I've got pretty good quality video (vga).
I keep gps sensor inside of trunk ceiling above metal framing and right under plastic cover (ledge behind rear seats), it works just fine.
The only difference from your setup is that I kept the radio, and hidden climate controls in the glove compartment.
My screen is 8" so actually I had to do it like that.
I took the power right from oem radio feeds (12V source, and 12V ignition signal).
The only problem I have is little hum in audio coming through the power, but I'll try with some in-line filters, we'll se what can be done about it.