for example in front bumper cfg :
slot 0.000 0.165 -0.214 0.000 0.000 0.000 3 ; F_bumper
attach 0x00000006 3 ; chassis->F_bumper
on car main cfg :
slot 0.000 -0.170 -2.143 0.000 0.000 0.000 3 ; F_bumper
you must add a line on the main.cfg
the number in green is the slot and must coincide on the 3 .In your case the number must be unique because you wanna make a new slot so put a number that dont already apear on the main cfg .
the yellow text is the chassis id must be the same that other parts
in case you use other main.cfg (another trim )
you must also add the second line,if you see on other part slot it need it
slot 0.000 -0.170 -2.143 0.000 0.000 0.000 3 ; F_bumper
compatible 0x00000006 3