A quick and easy way to do it is to buy a piezo buzzer from Maplin or your local equivalent and wire it to the output of the flasher, or if that doesn't work, two of them, one across each set of indicator bulbs.
I've also used one fed via a bridge rectifier wired across the two indicator live feeds.
However LR indicator units are something of a mystery to everyone and change part numbers without much warning now and again somehow independently of vehicle type.
When mine died a few years ago we tried two other that my local spanner chap had. One didn't work at all and the other was totally silent, and so I used the silent one for a week until the proper one turned up.
My advice is 'get used to it'. You've bought a car with character, learn to live with it.
I'm learning to live with the need for a new rear chassis member and the possibility of a complete new chassis and front bulkhead next year...
It's still cheaper than a car that age in decent condition.
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