my near side rear side light wont work the brake light works fine indicator fine. i have disconnected the holder and tried it in the opposite side and it worked fine all other lights are ok except this side light any ideas

The lamp/bulb is a good one isn't it?

Is their a local earth/ chassis connection? clean and secure?

Dirty bulb connections inside its holder/seat? - clean & CRC/WD40

Faulty connection in the closest plug/socket - unlikely.

Faulty wire to that connection?

No recent damage to the wiring loom in another place? rubbing / chaffing / mechanical damage to the loom?