Right turn signal causes both front and rear wipers to start.....

my 240 had this similar problem a month ago, the wiper start moving once i turn on the dim light and the wiper will stop once the main headlight is turn on. Finally , i find a grounding wire was loosen,its under the wiper water bottle. After this wire is fix ,all trouble was gone. So , I suggest that checking all  grounding wire first.

"Bill D. Drummer" ¼g¤J¡G

As someone else said, check the grounds first, but if you do have to replace the switches it's very easy, yes the steering wheel has to come off but for that you simply pop off the pad and remove one bolt, and the wheel pulls straight off.

I got a great deal on my 86 245 because the last owner could not figure out this same problem ... neither could his mechanic.

I asked here and was directed to a ground screw on the front wall in the headlight area where several ground lines come together and are connected to the body. I found the connection corroded and redid it ... problem solved.

The past owner offered me double what I paid for it to get it back .... I'm still driving it.

I'd look for a simular problem on your 740.

Good luck.