Right I've had a play in the garage with a mark 1 sierra rear wiper relay. This is a big orange one with "68 125 Relais Zus. Interm. W/W Heck" on the top. If you wire it up with a sierra stalk and escort rear wiper motor, then the motor will work from the front wiper controls both intermittently (approx every 11 seconds, not adjustable) and continuously (single speed only) :