Where to tax our car in Tarragona?

Hi. 

We are non-residential with an apartment in L'Ampolla.   We have a car that is registered in Tarragona (at the address of our solicitor's as this was the address on our NIE) and the annual tax is due.   We were advised to obtain a certificate of residency (can't remember the spanish for it!) from the Ajuntamento,  but they won't issue because we're not residents.  Therefore, as we understand it, we have to go to Tarragona to tax the car.

Can anyone help us with how and where we do this - and the documentation we would require to do so.

All help very gratefully received!

SueH