Just seen your question but hopefully you have sorted your cornering light by now!
In case not, for access there is a cover in the front wheel inner housing, so turn the wheel fully inwards for the side you are doing for more room, that cover slides out.
The cornering bulbs are type H7 55w for a GLA45 2015 I believe ( Drivers Manual will confirm ) and its worth getting a decent quality one like an Osram for longevity.
Mercedes will charge £63 inc bulb per side to replace.
All the best.