We have a 1996 BMW 5 and a 2000 Honda Odyssey. My husband is hearing impaired and he doesn’t hear the turn signal warning and forgets to turn it off. Is there a way to increase the sound so that he will hear it.
For many cars there are heavy duty and louder singnal units. I bought some years ago for my Cheverolet. If the dealer does not have them, perhaps an outo parts store does. The cost $10-$15 or so and can be installed in minutes.
Good luck!
Many people with many physical differences simply have to re-train themselves in the new manner needed to operate a vehicle. ( My wife was blinded in the right eye last year. Distance and right peripheral vision are vastly different now. )
The hearing impaired learn to relay more on their other senses, in this case vision.
He needs simply to remember to look at the indicator light on the dash.
You keep reminding him until this becomes habbit.
The old dog must learn new tricks. 
If the problem persists, perhaps fender mounted indicator lamps like many older cars had.
I don’t think a loud flasher (Is that one who wears a red plaid trench coat?) is available for either of these cars. Neither one has the old fashioned three pronged round flasher of old. I had an old truck with a bad muffler a while back. I put one on it. It was a good investment, much cheaper that a new muffler.