How do I register a car owned for 11 years ?

Is your driving licence American? French? UK? Aruban? Spanish?

U. K. 

Classic status is 30 years I believe.I think the biggest problem for you would be a Certificate of Conformity showing it meets European specifications.Its not impossible to register your car in Spain but I think it requires the services of a specialist to make it compliant.

Do you know about the current (post Brexit) rule for UK licence holders, that we can no longer exchange our licences for Spanish ones? And that we have to sit the Spanish driving test (theory and practical?)

Contact Leigh at https://www.spanish-number-plates.com/ 

I brought  a US car to Spain, a Honda (it was registered in the UK when I bought it). It was NOT a Euro-supplied model - the car type was never sold in the EU. It had no Certificate of Conformity.

In reality, there wasn't any problem. The challenge was assigning a pollution-rating for taxation purposes. My gestor used the pollution rating from another model (same make/same engine), and all was well.

Your 4.6 Mustang (Fox body ?) was sold here. There are a few around. I doubt there will be any problem, provided you have compliant lights, and a speedo with km/h readings. There is an issue with the VIN number - it MUST be stamped into the frame (usually on the scuttle behind the engine). A riveted aluminium VIN plate is insufficient. Some US models have 1 digit fewer in the VIN than the EU requires, so beware.  

Also, beware of any modifications - another one of my cars (a JDM Nissan) was given a hard time because it had non-standard HID headlights. They had to be switched out for the old-fashioned units.  

There are sometimes other minor differences between markets. The Nissan had side reflectors behind the rear wheels ... a useful safety item ... but NO, they had to be taped over. The Nissan was 29 years old when I registered it here - emissions were still tested.

The car must be matriculated here - a physical inspection. You will need to get it transported here.

The import tax is based on emissions and a "cadastral value" ... i.e. what the government thinks the car is worth. The cadastral value is, in turn, based on the original cost and a depreciation formula. Your car will be almost fully depreciated, so will be worth very little (as far as the government is concerned).  Your costs will, probably, be less than 2000 Eu, but suggest you contact Leigh (as recommended by Cheryl above).

Alternatively, there is a business near me - they brought in my two cars for me - they are a repair garage with a gestor attached, so they can deal with issues arising from the inspection process.  

I don't know about the classic status of the car but there are people in Spain who will advise you and can do the process for you for less than 100 euros. You may have to pay import tax, change the headlights etc. I used a company called Replate Mate. They organised the ITV and changing the lights and getting Spanish plates 

I did not have a good experience with this company. Details available if required.

Ok understood Cheryl sometimes that happened