I've actually be working on putting together a heftier version of one of these things. So far I've got 3 of the ~$13 (when on sale) harbor freight 1.5 watt battery maintainer panels and a 3" bilge blower with a motor scavenged from an old inkjet printer installed in it. I couldn't get the blower's original 40 watt motor to even twitch when driving it directly, and it barely spun when using an electronic speed control from an old cordless drill and a fat capacitor. The printer motor has the same size case and shaft as the stock motor but runs nicely with the solar panels in parallel, in spite of being a 24 volt motor.
I haven't gotten to installing it all yet, but I intend to put the panels inside at the top of the rear hatch glass (ideally I would get a bunch more of them and set them up louver-style) and connect the blower output to the existing air exhaust vents in the rear quarters of the car.

I think the custom solution is the way to go. A good ball bearing 12V computer fan can provide a fair amount of airflow without drawing allot of current.

I just crack the windows an inch. If thieves want in, they'll get in regardless, so don't worry about that. The only thing that I worry about is dust buildup inside.

Adding to Snax's coment...

How about working the A/C system backwards while on Vent from outside (as opposed to ReCirc). I don't mean using the A/Cs fan but maybe shutting off all to the vents but the top and adding a decent fan attached to the appropriate amount of solar panels which will be layed out on the dash?
The fan will need to be powerful enough to push air back out via the a/c ductwork.

It really isn't necessary to provision for the outflow of air in many (most?) vehicles, as there is already an outflow vent present - usually hidden and exhausting through the bodywork. Even lacking that, most cars are far from sealed. With a recirculating AC, the outflow vent still exhausts some air, so it's really more matter of getting the fresh air in. Just don't try to move air the other direction without other accomodation, as the outflow vent often contains a one-way baffle.

I have one of those fans but this one plugs into the cig lighter, so it will run continuosly despite clouds, etc, STILL -- it doesn't do much.

You don't have shade to park under, so why not make your own shade? Clip one of those aluminum sun shades under the wipers and leave the windows cracked an inch or so.

I was thinking that a powered solar ventilator hooked to the battery might makes sense provided you had a low power shutoff. I guess you could still have solar panels to top off the battery as well.

A similar tactic would be to use a small dedicated deep-cycle lead acid, or maybe a NiMH battery pack. Hard wire the pack to a charger/controller circuit of some sort so it charges up when the engine is running and discharges through the blower(s) when parked. That way you don't have to worry about having enough power left to start the engine... The blower(s) can run the dedicated battery flat and not touch the car's main battery. Where would solar panels fit into such a setup? I guess they could be set up to charge the battery/drive the blower(s) at all times, but they wouldn't really be necessary for operation.

A temp switch would be pretty useful as well, I would think.

Yeah, that occurred to me as well. The CRX is a rolling greenhouse... Even more so with a japanese market glass roof (not that I have one). One of the reasons I've resisted installing window tint is that solar heating occurs in the winter too. A 130?F interior sucks in the summer, but stepping out of the 20?F wind into a nice room-temperature interior is worth it in my book. Venting the interior during the winter would kill that heating effect.
A simple interior temp switch set at around 80?F would probably do the job, though a seasonal toggle switch could handle it too. Have it switch off the blowers and use the solar power to charge the two batteries.

I'm wondering about the possibility of a solar driven chimney type affair... a large flat one that sits over the windshield and covers the vents, black! Leave the vents open and the rear windows cracked, parked facing the sun, and theoretically, your black solar collector/chimney should start to convect and draw air up from the vents....