I'm not going to claim any expertise in the field of Horrorod building, but here's what I did. Big Lots (yep, the company that repossessed 100 DMC-12s in 1982 after DeLorean went bankrupt) was selling these 5 foot tall LED poseable skeletons for $37.50 each. My mother-in-law, ever the bargain hunter, spied these on a shopping trip one day and tipped me to their existence so I picked up a pair. In photos they look fairly realistic but are not anatomically correct, unless you've got Cyclops' disease and your eye sockets naturally glow red...
Despite one skeleton having two left legs - literally, we just folded that one up and out of the way - I'm happy with the quality and scare factor for the price. They pose easily, the LED eyes don't draw much power from the 3x AAA batteries encased in the skull, and they sit in slippery German square-weave vinyl seats without any need for ancillary support.
With a dapper cap for Mr. Horrorbones and a dress, scarf, and retro hat for the Mrs, we were in business. Ambient lighting inside the car comes from a 24-foot coil of "crystal-iced" orange halloween rope lights, strung through the 2002's interior with abandon. You'll notice in the first photo that I also turned on the parking lights for added effect, but not before throwing a cheap battery tender in the trunk and plugging the whole thing into my garage via extension cord.
The overall effect is quite spooky at night, albeit not so much during the day. But hey, at least the 1960s legacy plates finally showed up!