Locking issues on my fiesta mk6 2006!!

Hello! 
 

I bought a mk6 fiesta (56 plate) last month and I’ve just had an issue arise with the locking. It started with the door open light staying on the dashboard, and the car would unlock every time I locked it. We did some googling and figured if we flick the door lock mechanism thingy then we can get the light to disappear, which then allowed me to lock the car.  Failing that I would hold the key for a few seconds when I lock the door to stop it unlocking again. It’s probably worth mentioning that the buttons on my key haven’t worked since I’ve had it so I’ve had to lock and unlock the car using the actual key. 
 

In the past week a new issue has arisen in that the passenger door now won’t lock with the rest of the car. So when I Lock my drivers door, I have to go round to the passenger door and lock it from the inside lock.  We’ve seen some things saying to check the wiring closest to the door hinge as this may be an issue if the door has been slammed too many times, but we can’t figure out how to get to this wiring. 
 

im not really sure what I should be checking/replacing and I really don’t want to have to pay a garage, (if I can even get to one given the current state of the world) so any help would be much appreciated! 
thank you 😊