No clarity has been established in this thread. Apparently some insurers don't count the fitting of a tow bar as a modification, but others do. Where does that leave the customer? Unless you ask your insurer about their policy on tow bars or any other modification, then you will never know until its too late.
Regardless of what may have happened in the past, insurers do change their policy limitations over time, so you should not assume the policy you have this year is the same as last years. Your insurer is required to advise you any changes they make to their T&C at renewal, so don't cast the letter aside, read it and if there is anything you need further explanation about take it up with your insurer.
I really cannot understand why so many of you are so put out about the need to declare modifications to your insurer. Surely its so much better to declare and be told it wasn't necessary rather than finding you have claim refused because you didn't declare a mod.
The problem falls on both th einsurance companies and the customers, not knowing what each needs.
I suggest flooding the insurers with information, and when they decide its too much they might produce a consistent industry wide policy with clearer definitions of what customers need to tell them.
Happy New Year.