speeding ticket in Spain

In trying to find an answer to your specific question, I lifted this:

"To enter the query should be :

1.Tax ID number , NIE , ID , passport , residence permit , residence permit or number of the person accused of a resident community .

2.File number . The file number is the number of complaint and is printed in box number 1 in the form of condemnation or paragraph < > the written notification sent the Catalan Traffic Service . You must enter all the numbers in a row, without bars or dashes.
"

From here:

http://www20.gencat.cat/portal/site/OVT/menuitem.8d9f3f7e23c1cd519e629e30b0c0e1a0/?vgnextoid=5efafd563a1bb110VgnVCM1000008d0c1e0aRCRD&vgnextchannel=5efafd563a1bb110VgnVCM1000008d0c1e0aRCRD&vgnextfmt=detall2&contentid=df08ba2cd622c110VgnVCM1000008d0c1e0aRCRD&idTipusAction=1&idAction=2&id=7f08ba2cd622c110VgnVCM1000008d0c1e0a____&nomAccio=1&meta=#apartat2pas1

which is one of the sublayers of the site you noted.

It looks like #2 is your answer.

Update. We have been trying to figure this out for well over a month now. We contacted the Spanish Embassy for help and I've asked Spanish speaking friends to translate phone messages.
I finally resolved it by trying to remember what info my husband had to give the rental company. I kept thinking it was a passport # and picking the "passport" option in addition to the ticket number. The system could never find the ticket. Well he also gave his US driver's license. I picked "altres" for identification and had to put in the driver's license # without the preceding alpha character and viola!
The worst part of the whole thing is that if we would have paid it in the first 30 days, we would have gotten a 50% discount. Well at least we can go back to Spain without fear of my husband being arrested.

I applaud you, Christine, for your diligence in following this up.