The first Aire referred to by Runnach is the preferred location, but very busy, vans will be about a metre apart, so be there early to mid morning to increase your chances of getting a spot. We have succeeded twice, but failed twice.
The other Aire under the motorway near the stadium named arena Donastia is ok. We have stayed twice, but word has it that vans are less secure there. We have parked if there have been several others there. Bit slopey but otherwise fine. Buses close by otherwise a fairly steep walk down the hill, then a flat but longish walk into the old town (similar distance to other Aire). Once down the hill it is an easy dedicated cycle path to town. There is a service point at the northwest end of the car park, right under the bridge.
Davy