I've been digging based on some of the answers/comments here and here's what I've come up with.
Andy's suggestion that the light was lowered after re-entry, possibly from the parachute compartment sounded pretty good. This picture of a landed capsule had me suspecting the white object might be some sort of beacon, but other pictures showed it to be part of the parachute. Thinking a little more, it seems unlikely that anything except parachute would be put in the parachute compartment, given that a tangled cable could have life threatening implications for the crew.
This artist's impression from an article about Soyuz 23's landing seemed to support the idea of a beacon hanging from above. A close up of the compartment in question shows a strobe light and a whip antenna, but the light doesn't appear to be extendible, making me think that the artist misunderstood the difference between the extendible antenna and the static light. Other pictures of landed capsules show the light still in place after landing. Furthermore, I couldn't see any trailing lines down the side of the capsule in this image which shows the beacon.
Soyuz: A Universal Spacecraft lists a 'beacon' on the underside of the capsule, but doesn't go into any more detail (I don't have a full copy of the book).
Finally, I came across this image that seemed to settle it:
Click to zoom and you can see a beacon/bulb still attached to the underside of the descent module. Perhaps it didn't release properly this time? The length of the cable seems to support the idea that it is stowed under the heatshield. It still isn't entirely clear where it plugs in to the capsule or how it is stowed (I'd hazard a guess at tucked into one of the soft-landing engine wells or as shown in the image), but I'm happy with this as an an answer.
(I was previously working on the assumption that there might be 2 beacons (one brighter, one dimmer), but on closer examination of the video, the flashes do seem to be emanating from the same spot, implying a single beacon.)