enjoy your web page.
Here are a few tips 4u
-remove the kick panel beside your left foot, or as in case of passenger - the right foot. Behind these you will find plastic panel covers (plugs) that were installed to block the holes in the metal. When originally installed the plugs had a thin foam strip on them as a seal. Removal of the drivers side is easiest, the passenger side one will mean pulling out the cpu- highly recommend disconnect the battery first.
I think the holes are where the vents were located on the 240's. (Volvo figured 740's did not need this ventilation and so stopped providing it - gee- ever been in a 740 when the air does not work - the car is very hot as there is no way to get rid of the heat at floor level soaking in from the engine compartment).
Most 740's will leak water around these plastic plugs and will soak the floor (which will help to rot them out) and if one lives where the air temp goes below freezing one will see frost / ice form on the inside of the car's windows caused by the wet front floors. Remove the plastic plugs, clean off the remains of the foam, coat the sealing edge of the plug with silicone and reinstall, then smear on more silicone sealant around the edge of the plug. Now is a good time to put a very light smear   silicone grease on all the contact points for the cpu. I know of numerous people needing a new cpu because of this water leak, sweating, and mild corrosion of the cpu contacts.
I actually pulled every electrical connection apart on the car, cleaned them, then put some of the dielectric silicone grease
on each - avoids electrical problems. As far as the cpu is concerned, with corrosion in any of its under hood sensor wiring harness connections causing intermittent electrical signal will cause the cpu to act up or fail. The normal dealer fix is to slap in a new / rebuilt (from scrapped cars that worked) cpu while not overcoming the real reason the cpu failed.
Water leaks in rear tail light lenses - dealer fix is to drill small holes in lower edge of lens so water coming in will go out.
You have to remove tail light assembly and reinstall making sure the seals on body is good.
Electric window switches - you got the right idea. Those rocker contact strips with the little silver contacts can be repaired,
use silver solder and build up the contact area - only a drop of silver is needed, then you file the contact area flat. I have re-bent the rocker strip as it seems to heat up when in use and loose it's shape.
I have other tips but this is enough for now. I have had to learn how to repair the car myself as the local dealer is very dishonest and milks both the customers and the company.