Hi,
Not seen many racks on the backs of cars out here except on motorhomes when they always seem to have a reflective board as part of the installation. I would imagine therefore that the board is a legal requirement.
I would also suggest that the same rules apply as in the UK whereby no lights are obscured by the bike when fitted to the rack and that the registration plate is clearly visible.
Given that the GC seem to be trying to clear Spains National Debt by clamping down on motorists for the least little thing these days I would say better safe than sorry. Dont give the bu**ers an opportunity to dream up some new non-compliance rule!
fb