What ever you do DO NOT try to disassemble the ratchet/roller mechanism. There is a fairly long, tightly wound mainspring that will come flying out at you with sharp edges (don't ask how I know this).
If you want to try to engage the inertia lock you have to try to pull the belt out quickly, sometimes with a slight hesitation before pulling further, that may engage the inertia catch. Once caught, then you have to maintain steady pressure to keep it engaged.
If you do decide to pull the ETR mechanism (seatbelt roller assembly) from the "B" pillar for inspection, remember the is a pyrotechnic charge to deploy the ETR in a level 1 crash. When you disconnect the connector to the squib, the squib should automatically shunt, rendering it safe. Upon reassembly you must hear the audible click or the ETR will be non-functional and you'll get the SRS MIL.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.