Sorry, but is it not a flasher relay like one used often in motorcycles? It is wired in series between power supply and switch (whichever the 30 and 31b are - I do not remember these numbers from convention). You may want to verify this so that you do not blame me if your car burns down, but all you need to make it a 3 pin is to ground the third pin on the normal 3 pin sender... I think the two connections 30 and 31b shold be one to fuse and the other to switch...
It is No. 12 on the attached. Maybe do not look for it, just make it 3 pin, no?
Pawel