2006 Vauxhall Vectra offside rear door locked shut even when unlocked.

If it doesn't open manually from inside or outside you will have to pull trim off and check linkage / lock and wiring .

How do you get to the screws at the bottom of the door panel when door is locked shut

A long shot but worth a try .If you open front door and can get to the wires passing in through rear door frame from pillar , try moving them whilst operating remote in case broken wiring .I know what you mean regarding trim removal and getting at any screws / clips holding it on . Seen some where we had to tear trim to gain access .The only other suggesting is to get a thin wire ,like a coat hanger with a hook on the end and put it down side of window to try and hook on to door rod/ linkage inside door and open that way .A lot of door locks are shielded which doesn't help .

If I remember correctly the panel is held on mostly by clips around the edges, which can be unclipped by - I normally use a small Trowel - insert the Trowel between the Door and the Panel and apply light pressure and work your way around the edge of the panel. There are a few screws holding on the door pull. If you tilt the seat base forward and then the backrest down you should be able to work your way around the panel. It will then need to be lifted slightly upwards,If prior to removing the panel you lock and unlock the Car while holding your Ear close to the Door in the Lock area you should be able to hear the Lock mechanism moving. Also I seem to recall a similar problem posted a few weeks ago so you could refer back to that as well. Best of luck with it.