If you are using a skin for both cab sizes and each has it's own main sii file (_a and _b), then you need to make both _a and _b here as well. 

SII NAMES:

The names of the sii files are important. The first part (example_a) must be exactly the same name as the main paintjob sii file name. 
The second part (.painted) must be the correct name for the model part. I believe the name is taken from the .pmd name. 

TOBJ LOCATION:

As with all tobjs you can place them where you wish. I find /skinname/accessories/ to be the "neatest" option. 
You can of course alway link to the same tobj file where appropriate. 