Re: Chevy Express Rear Window Defrost
Most factory-installed rear window de-icing systems tend to have the timer and current relay built into the dash mounted switch. I'm not familiar with the Chevy van version though---I'd look for one with this already installed and see how its handled as a new option.
From there I'd look for a wrecked van near your same year and cut the pig tail lead for the rear window defroster system, perhaps the switch would be there too. If not the dealer would be one choice, eBay another.
If you choose to run your own wire I'd use nothing less than a 10 gauge wire since you're running maybe 30 amps 20' or more. I can direct you to a variable time delay relay good for a max of 10 amps, could be used as a pilot switch for a heavier relay. It could also function to set the off time after start, all solid state and about $30 for most versions.