You can buy canned boat horns, such as used to be found at sports events. I think they were outlawed there because of potential hearing damage to fellow sports fans. It is no joke to toot one next to someone's ear.
https://www.amazon.com/canned-air-horn/s?k=canned+air+hornBut you have to ether stick them out the window (inadvisable in the subject situation) OR rig a remote trigger puller from the engine compartment to inside the car.
I got one for emergency signaling. The effers are effin
LOUD!
If I'm remembering correctly, aircraft electrical systems are 400 Hz and 24 volts. The 400Hz is because transformers and other ironbound things need less iron core (are lighter) for 400 HZ than for 60 Hz, and hum filtering is far less difficult at 400 Hertzes than at 60 of them.
For the landing lights, incandescent ones would work on DC.
Corrections invited.
Terry, 230RN