This is more of the instructions on how to _do_ a half-baked idea than the idea itself. Forgive me, and read on.
To make a "life-sized" rubsign, (I.E., the liquid-crystal based stickers on later transformers that had to be touched to reveal whether the bot was Autobot or decepticon), you need several
difficult-but-not-impossible to obtain components:
a. Some sort of analog Liquid Crystal window - they make actual windows that do this, and I've also run into instructions on converting dead monocrome displays to analog - but some sort of electrically-variable opacity screen is needed.
b. Red (or purple, if such exists) Bioluminescent Polymer strips. The stuff is sold to ravers, fashion designers and whatnot. I think they have the right color of red, but `Con purple may be tough, if not impossible.
The rest of the materials are fairly easy, you need a 12-volt DC-AC transformer for the BLP, a second power-supply for the "window", some plexiglass to cover the entir thing with, and odds and ends: exact-knives, wire, something to drill through your car with, the lot.
The idea is that you begin by setting up your control electronics under the hood, run wires through a hole you put there, the lay down the sheet you have made for your logo. Around the rim of the sheet (below the "lip" of the window), you should construct some sort of touch sensor - I was thinking some combo of foil and high-carbon foam to act as a resistence sensor. Above that, place and wire-in the LC panel, and then frame and plexi over the entire thing.
The reason I wanted to use a resistance sensor would be simple - the thing would be triggered by a hysteresis circuit. Hysteresis (I don't know if I'm spelling ir right) circuits turn on at the right stimulus, and then stay on for a bit, only turning off after a while, and ignoring further input until they're off. I can't build one, lacking any electronics experience, but I know how they work, roughly.
It would then work like this: There's a black panel on your car that sits there, when you tap it (fairly hard), an autobot or decipticon logo should SHINE out from within it for a few seconds before it slowly goes dark again. Weatherproofing this project would be the hard part, but it wouldn't really be half-baked if it were pratical or reasonable, would it?