True that. Listen to LK!
Doesn't matter what you use. My only preference is stranded and between 20-14gauge. Other than that, who cares? Make sure it's easy to work, won't break with normal movement, and won't make a mess of you cavity. Nothing ticks me off more than a messy electronics cavity.
Oh yeah, and as for my own answer to your orignal question: Yes, by all means use the 20ga solid core wire, it makes no difference. Just try not to sling the harness around by the wires or anything. Solid core WILL break at solder points if it moves too much.