You talkin' about:
hiding wires for Radar Detectors?
the lighter adaptor gay neon tubing people buy from Autozone?
misc. devices (amp, computer, tv, whatever) in your trunk/headrest/dash?
For #1, drill holes in the dash?
For #2, don't buy them.. no matter how cool you think they are.
For #3, I always hide them under carpet in plastic conduits. Fasten them with Zip ties around other harneses leading in the same direction, or remove/restitch seat covers.
Engine bay - your on your own.. that s**t's always tricky - with heat problems and/or short problems around every other turn.
Keep it off the hot spots, and away from your suspension..
Pinching/melting a poorly routed wire and killing your whole setup sucks.