If I am not mistaken, late '80s 7-series Volvos had eco-friendly wiring
harness that deteriorate over time. That deterioration results in
intermittent and odd electrical problems with multiple systems in the
vehicle. From the threads I read, people with late '80's 7-series vehicle
have had very good success by replacing the wiring harness in the engine
compartment.

I don't sell them but you can find them by Googling "740 wiring harness".
There is even a site that shows how to do it.

I hope that helps.

GMan in Raleigh