Not sure if this will help answer your question, but maybe. The visual indicator, is
not to remind someone they are towing , if they are not aware of that , they should not be driving. Especially as all drivers should be frequently checking ALL mirrors! It is to tell you that the trailer indicator lights are working.
The visual or audible warning must tell you that your trailer indicator lights
are working, rather than bulb failure light, which tells you they are not.
Check your owners manual to see if you have a visual or audible warning system. If not, time to get one wired in.
Hope this helps.
P.s Just noticed trailer lights are LEDs. This may be the issue. People cleverererer than me can tell you why
@Saint.V8 ,
@holidaychicken ,
@wammers and @ others who might know
2nd addit: Considering where you work/ed , surely someone there would know the answer to this, and can give you hands on help?