I have a 1990 volvo 740 that i bought in the summer.
Now the rain season is upon me it has flooded twice.
The water pools in the back passenger foot well also travels to passenger foot well
I have also noticed water in the wheel well in the trunk
could it be the lights leaking and traveling through the car?
The heater has also only started working at full blast but I have bought a motor resistor online to hopefully fix that issue or it could be related?
any help would be great!

 

Your blower motor switch suggests the problem is forward, not behind.
It's possible that something as simple as the cowl drains are plugged and water is backing up into the fresh air entry, which is in the cowl, on the passenger side.

 

My 940 has leaked through the windshield, leaving a puddle on the front passenger floor. I've also had the A/C condensate drain plug up with crap and flood it just as bad, if not worse. A new windshield fixed the leak, and occasionally blowing out the condensate drain with compressed air stopped the A/C issues. Of course, the 6 gallons of water that came out flushed the debris out pretty good, lol.

Obviously, condensate won't be an issue this time of year, unless you live in Australia or S.America.

 

Is the car parked facing slightly downhill?

I had a customer (years ago) - they would park on a slight downhill. Don't remember if it was a 740 or 940 sedan. During heavy rains - the trunk lids drains would fill up - and flow through vents that are at the top of the recess for the trunk lid. Water would go in those, down the back of the backrest and into the floorboard. Figured it out after removing the rear backrest - and there was water on the ledge above the floor.

Solution was to not park facing downhill.