However, this would appear to mean that rear engined cars are marginally more efficient than front engined, since bulkheads reflect infra-red more and grilles transmit it more.

Easier said than done. The reason you put thermal radiators on your spacecraft is to dissipate heat that would otherwise fry your electronics. But these radiators can absorb photons as well as emit them, and you have a major source of infrared photons called the Sun somewhere behind your spacecraft. If your radiators are in front of the spacecraft, they don’t collect these photons because they are in the shadow of the spacecraft. In addition, you need an antenna on the back side of the spacecraft (so it can phone home), and you will probably want a particle detector of some kind (because the people who are backing your mission will want to know what the solar wind and solar energetic particles look like).

I’m sure there are ways to do it, which may not have been understood when the Pioneers were built. But it’s not a trivial thing; there are engineers who specialize in spacecraft thermal design.

But I think I know what NASA did. It launched the first boomerang space probe!

Because of this radiation emission, at 10.000 years from now, Pioneer will return to the Solar System!!!

Next winter don’t forget to turn off your headlights before trying to push your car out of that snowbank; you don’t need any extra resistance!