http://www.tumbit.com/how-to-guides/articles/88-tow-bars-in-spain.html

http://www.discovercostacalida.info/law-on-tow-bars/

The above are a little dated but one will glean from them that is the tow bar is part of the ITV examination if it is in place. if "Factory Fitted" and the paperwork is at hand to verify this you should be OK however one fitted after you drive off the forecourt in the UK is illegal here and you will have to change it to a Spanish model. Then return to the ITV station with the new fitted tow bar and paper work that comes with it. The fitters must be approved apparently. If you are stopped without updating your docs re the new tow bar you may face a fine.  :(