When you speak of the "locking/cancellation mechanism", do you mean the plastic ear-shaped piece on top of the switch (closest to the driver when installed) which contains the detents for the RH and LH turn positions, and has the tabs that get hit by the finger on the steering wheel to cancel the signal? If so, it just pries off the switch body with a wide flat-blade screwdriver. Is that the only problem with your turn signals, that they won't self-cancel?

Mmmm. When you get the cancelling cam off the switch, notice and remove the little brass-wrapped plastic shuttles that actually slide up and down to complete the various circuit paths in the switch. Clean and inspect all the contacts, on the shuttles and in the switch body, then apply a thin film of Sil-Glyde grease (not the thicker silicone dielectric grease) to lubricate the shuttles and reinstall them. Be sure they go in the right way round; take careful note of how they're installed before you remove them. Test out the operation of the switch carefully before putting the column back together; if it turns out you need a whole new switch, you won't want to haul it all apart again!

You may want to see this thread for techniques to fish the signal switch wires up and down the column.