Heater/towbar electrics fault

I have had a towbar fitted to my P38 2.5DSE by a reputable towbar installer last week and now when I plug in my 12S plug (grey one) from my caravan it illuminates the book symbol for the heating system.

Any ideas why this would happen?

I am going to speak to the installer tomorrow but I am not keen to let them loose to cause havoc with my P38 and would prefer to speak to them armed with a little knowledge of what might have caused this problem.

it depends where they have taken the power from.It MAY be the van is pullin too much juice on a single circuit.P38s are strange things.

Power for the 12S socket should be taken direct from the battery via an in line fused cable to a double pole relay that is best located near the near side rear light cluster where the other connections can be made to the relay