Classic Side lights not working on one side

After I said I hated electrics with my last electrickery issue, Red Rover has decided to play up some more.

I have side lights on one side (the right) but not the left. Front and rear lamps both not working.

Everything else works on the left, including the other filament in the bulb (the brake light) at the back. The fog and indicators, and the reverse lamp work on the left.

Am I right in thinking:
1) most likely a wiring issue?
2) most likely near the switch?

Logic being:
New bulbs don't fix it, and it's two lamps operated by the same switch. If it were a fuse, I'd expect to have more lamps not working. Right?

Solved it, it was corrosion in the front bulb holder, all cleaned up and working again now. I checked the back, it was fine, figured the front would (it was cold and dark in my defense). I'll leave this here for anyone else who has the same issue and hates electrickery!