Fan heater not working

Turn on my fan and all that happens is the radio or the interior light dims….is this earthing matter or something else.

Where should i start?

Ok got it working…. for people in the future who have the same porblem -
the fan was jammed so i took off the plastic guard in the engine bay (by the wiper mech), i then checked to see if the fan would work off a direct feed so i connected the battery to the fan and it made a bit of clunk noise. So then put hand in the fan and found it was jammed. Gave it a light twist and it came free. Rehooked it up to the battery and it run, albeit alittle grindy Sprayed some WD40, reconnected the wires and put the cover back on and bobs your uncle!

best to take it out and clean it up, or it will sqeak like mad in 6 months time!