Thanks to RMD Audio of Wellingborough, I now have a pair of 6 inch Boston Acoustics coaxial door speakers, and my old 10 inch Kenwood Sub and Kenwood amp in the boot.

It sounds nice and loud, and they did a lovely job.

Apparently, the door cards were only used for 6 months, and there are are no fitting aids for the TDi 1.4 03 Polo.