You could run a cable along the roof guttering, down the door jam then squeeze it through sort of behind the wing mirror and into the engine bay and then through the wiring grommet on the engine bay bulkhead behind the glove box. Then where ever you like from there. Downside, getting squashed when door opens if not routed properly or cable too thick.
Alternatively, you can open the rear door and carefully pull down the rubber weather seal from the body work, but only enough to fit the cable through, then through the trim. Push the seal back up and you should never tell the difference. Best to use some glue (nothing crazy like rubber cement) to reseal it as water can technically travel down the cable and get to places it shouldn't. You can then run the cable through the head liner to the front or under the carpets and under the door trims.
If you go with the second method, just be very careful pulling the seal off the bodywork lip, maybe use a flat head to help it. And make sure its a good seal when you put it back. If in doubt, put it back to how it was. This method works and I have had a rear mounted camera cable running through like this for several years and no water gets in and no dampness under seal. The only real downside to this method is it puts one hell of a kink in the wiring and you are really limited to very thing wires only. If your beacon uses a thick wire, you're gonna have to either go for the first method or drill somewhere.
Can take pictures tomorrow if you like.