I used to build these for the RCMP in my small home town when I was a youngster. Darned simple: a Tridon Electronic Flasher with alternating outputs and an automotive-grade SPDT toggle switch (25A contacts).
The particular Tridon Electronic Flasher I used was similar to the standard signal light flasher (two terminals) but this one had 3 terminals. One terminal was Normally Closed, the other was Normally Open. The On/Off toggle switch disconnected the load from the flasher NO terminal and instead connected it to the input Hot terminal of the flasher. The flasher does not operate if there is no load connected to the NO output terminal - thus the load connected to the NC terminal always had power.
These Tridon flashers were sheer magic to me when I was young. The relay coil has a bifilar winding and a large non-polarized electrolytic capacitor. Circuit operation is such that the relay contacts are OPEN until the capacitor charges. When the contacts close, the charge on the capacitor keeps them closed for the appropriate time period. Wire size of the windings (DC resistance) and the capacitor value determines the time constant. Brilliant engineering!
The reason I was making these for the RCMP is that the little potted modules supplied for their headlight wig-wags had a distressingly-short lifetime. My solution was much less expensive and much more reliable - the Tridon Electronic Flashers have a current rating of something like 25 Amps. All that current is switched with insanely-reliable relay contacts instead of wussy little bipolar transistors. Keep in mind that this was the mid-to-late 60's.
Tridon Electronic Flashers are still available. Although I don't recall the part number of the device I used to use, I'm thinking that the HD13 might be the correct part. But I don't have one at hand to check.