I've wanted to put an active shortwave antenna on my car for ages, but
didn't want to start messing with building one.

Years back I got replies from Universal, Gilfer, EEB etc. saying that
they never saw such a thing - but they might carry one if available.
In particular, several resellers said they might have liked a well
made, overload-resistant, mag-mount unit with a DC-RF splitter.

I might add I would have loved something that
- dampened both FM and Medium Wave (never seen one, not even plans for
one, and both bands are a big problem in my area)
- worked well on 12 Vdc (not 24)
- cost less than $1000.

I even talked to an active car radio antenna maker who told me the
design changes were trivial. Their best active car antennas had
separate AM and FM channels, so they could have moved the AM passband
up (they had standard cutoffs at 100 kHz and 5MHz, as to cover
longwave too) and left the FM channel unbuilt. The problem was, they
were an OEM, never did a production run of less than say 10k pieces
and weren't enthused about selling into a specialty channel with
ridiculous volumes and 100s of distributors in 3 continents!

As it happens, if you want something MIL-spec running on 24Vdc, have
$1000 to burn and you don't mind having to drill holes in your car,
and to cope with big signal above and below HF, you're well served.

I did some searching recently and still came up empry handed.

Is there something I have missed?