Well, the connector finally came through, but the GBA and SP don't seem to want to connect. One will say "Waiting for other player" whilst the other says "You need to be connected to do this"...
I'm gonna borrow my friend's SP to see if that works. if it doesn't, I'll get a refund from TulipBuds who sold Pokémon Red to me. I was really looking forward to this, now it's all gone wrong, sigh.
