jcfd55:
Yes it is but I’m using the 12v plugs anyway as I don’t have the smaller plug
Are you sure you’re connecting to 12V? In my (limited) experience the ■■■ lighter sockets on trucks are generally 24V, with 12V and 24V being available using a Hella connector (i.e. “the smaller plug”). Sorry if I’ve misunderstood your response.
Even if the adaptor is rated for 24V, it will still have to dissipate more heat if used at the higher voltage which could well lead to premature failure (especially with cheap Chinese electronic components).