Spanish law states that unless an accessory was already fitted when the car left the factory or fitted by a main dealer, with documentation to prove, then it is illegal and is an automatic ITV (Spanish MOT) failure - this also applies to the spotlights. Any such accessory has to be shown on the Spanish version of the V5. My towbar could have caused a fail sheet, but for the fact that it was already fitted when I imported the car and there was nothing to prove that it was not factory-fitted (it's a genuine Nissan one that I fitted myself in the UK). I fitted the bullbar after re-registering the car as an import, hence it is not shown on the V5.
The Spanish are very hot on any changes from the original spec (I heard of someone who got a fail because he had rubber door-edge protectors fitted, although I somehow doubt that one!)
A friend of my daughter had his Shogun fail because he had a towbar and ball fitted - he drove around the corner, got his spanners out and removed the ball, drove the car back 10 mins later and got a pass! Apparently if there is no ball or hitch fitted you cannot use it for towing (which is also illegal in Spain unless the trailer/horsebox/caravan has also been MOT'd) so it was allowed. A ludicrous state of affairs - he drove home and refitted his towball, safe for another year, I will do the same with my bullbar.
The only time you can come unstuck is if you are involved in an RTA and the Policia/Guardia are involved, they inspect your V5, which you must carry at all times, and anything fitted to the car which is not shown on that V5, you will reported for and fined.
It's a minefield!