Many years ago I purchased a generic aftermarket delay switch from JC Whitney and wired into the existing wiring at the dash switch. I recall it being a little black knob that you turned back and forth to increase/decrease wiper delay. What I can't remember exactly is whether it was ADDED to the existing pull knob or a REPLCEMENT. I'm pretty sure I left the original knob in tact and added the delay into the wiring - so what you would do was turn on the wipers with the original knob then adjust the delay with the little turn knob. It came with instructions for wiring it in - not real difficult. It worked great.